GPF: xpt_done_process got invalid ccb_h->path->bus pointer

-CURRENT as of 24cd2796cf10211964be8a2cb3ea3e161adea746

This race can be triggered on a host with 1394 enclosure attached by
using the following loop:
  while true; do
    kldload sbp; kldunload sbp
  done

Fatal trap 9: general protection fault while in kernel mode
cpuid = 13; apic id = 0d
instruction pointer     = 0x20:0xffffffff8038be3a
stack pointer           = 0x28:0xfffffe0269e07b30
frame pointer           = 0x28:0xfffffe0269e07b60
code segment            = base 0x0, limit 0xfffff, type 0x1b
                        = DPL 0, pres 1, long 1, def32 0, gran 1
processor eflags        = interrupt enabled, resume, IOPL = 0
current process         = 41 (doneq0)
trap number             = 9
panic: general protection fault
cpuid = 13
time = 1616639524
KDB: stack backtrace:
db_trace_self_wrapper() at db_trace_self_wrapper+0x2b/frame 0xfffffe0269e07840
vpanic() at vpanic+0x181/frame 0xfffffe0269e07890
panic() at panic+0x43/frame 0xfffffe0269e078f0
trap_fatal() at trap_fatal+0x387/frame 0xfffffe0269e07950
trap() at trap+0xa4/frame 0xfffffe0269e07a60
calltrap() at calltrap+0x8/frame 0xfffffe0269e07a60
--- trap 0x9, rip = 0xffffffff8038be3a, rsp = 0xfffffe0269e07b30, rbp
= 0xfffffe0269e07b60 ---
xpt_done_process() at xpt_done_process+0x12a/frame 0xfffffe0269e07b60
xpt_done_td() at xpt_done_td+0xf5/frame 0xfffffe0269e07bb0
fork_exit() at fork_exit+0x80/frame 0xfffffe0269e07bf0
fork_trampoline() at fork_trampoline+0xe/frame 0xfffffe0269e07bf0
--- trap 0, rip = 0, rsp = 0, rbp = 0 ---
KDB: enter: panic

__curthread () at /home/freebsd-current/sys/amd64/include/pcpu_aux.h:55
55              __asm("movq %%gs:%P1,%0" : "=r" (td) : "n"
(offsetof(struct pcpu,
(kgdb) where
#0  __curthread () at /home/freebsd-current/sys/amd64/include/pcpu_aux.h:55
#1  doadump (textdump=textdump_at_entry=0) at
/home/freebsd-current/sys/kern/kern_shutdown.c:399
#2  0xffffffff804c7d2a in db_dump (dummy=<optimized out>,
dummy2=<unavailable>, dummy3=<unavailable>, dummy4=<unavailable>) at
/home/freebsd-current/sys/ddb/db_command.c:575
#3  0xffffffff804c7aee in db_command (last_cmdp=<optimized out>,
cmd_table=<optimized out>, dopager=dopager_at_entry=1) at
/home/freebsd-current/sys/ddb/db_command.c:482
#4  0xffffffff804c782d in db_command_loop () at
/home/freebsd-current/sys/ddb/db_command.c:535
#5  0xffffffff804cafb6 in db_trap (type=<optimized out>,
code=<optimized out>) at /home/freebsd-current/sys/ddb/db_main.c:270
#6  0xffffffff80c5c754 in kdb_trap (type=type_at_entry=3,
code=code_at_entry=0, tf=<optimized out>, tf_at_entry=0xfffffe0269e07770) at
/home/freebsd-current/sys/kern/subr_kdb.c:727
#7  0xffffffff810bf97e in trap (frame=0xfffffe0269e07770) at
/home/freebsd-current/sys/amd64/amd64/trap.c:576
#8  <signal handler called>
#9  kdb_enter (why=0xffffffff812b664a "panic", msg=<optimized out>) at
/home/freebsd-current/sys/kern/subr_kdb.c:506
#10 0xffffffff80c0faf2 in vpanic (fmt=<optimized out>, ap=<optimized
out>, ap_at_entry=0xfffffe0269e078d0) at
/home/freebsd-current/sys/kern/kern_shutdown.c:907
#11 0xffffffff80c0f883 in panic (fmt=0xffffffff81e9a738 <cnputs_mtx>
"\202;'\201\377\377\377\377") at
/home/freebsd-current/sys/kern/kern_shutdown.c:843
#12 0xffffffff810bfdd7 in trap_fatal (frame=0xfffffe0269e07a70, eva=0)
at /home/freebsd-current/sys/amd64/amd64/trap.c:915
#13 0xffffffff810bf264 in trap (frame=0xfffffe0269e07a70) at
/home/freebsd-current/sys/amd64/amd64/trap.c:212
#14 <signal handler called>
#15 xpt_done_process (ccb_h=0xfffff80102f2f000) at
/home/freebsd-current/sys/cam/cam_xpt.c:5419
#16 0xffffffff8038e0f5 in xpt_done_td
(arg=arg_at_entry=0xffffffff81bc4980 <cam_doneqs>) at
/home/freebsd-current/sys/cam/cam_xpt.c:5544
#17 0xffffffff80bc9a60 in fork_exit (callout=0xffffffff8038e000
<xpt_done_td>, arg=0xffffffff81bc4980 <cam_doneqs>,
frame=0xfffffe0269e07c00)
    at /home/freebsd-current/sys/kern/kern_fork.c:1077
#18 <signal handler called>
(kgdb) up 15
#15 xpt_done_process (ccb_h=0xfffff80102f2f000) at
/home/freebsd-current/sys/cam/cam_xpt.c:5419
5419                    sim = ccb_h->path->bus->sim;
(kgdb) print *ccb_h
$1 = {pinfo = {priority = 1, generation = 11, index = -3}, xpt_links =
{le = {le_next = 0x0, le_prev = 0x0}, sle = {sle_next = 0x0}, tqe =
{tqe_next = 0x0, tqe_prev = 0x0}, stqe = {
      stqe_next = 0x0}}, sim_links = {le = {le_next = 0x0, le_prev =
0x0}, sle = {sle_next = 0x0}, tqe = {tqe_next = 0x0, tqe_prev = 0x0},
stqe = {stqe_next = 0x0}}, periph_links = {le = {
      le_next = 0xffffffffffffffff, le_prev = 0xffffffffffffffff}, sle
= {sle_next = 0xffffffffffffffff}, tqe = {tqe_next =
0xffffffffffffffff, tqe_prev = 0xffffffffffffffff}, stqe = {
      stqe_next = 0xffffffffffffffff}}, retry_count = 0, cbfcnp =
0xffffffff826fdfe0 <sbp_cam_scan_lun>, func_code = XPT_SCAN_LUN,
status = 1, path = 0xfffff820d9c10fa0, path_id = 6,
  target_id = 0, target_lun = 0, flags = 2048, xflags = 0, periph_priv
= {entries = {{ptr = 0x0, field = 0, bytes =
"\000\000\000\000\000\000\000"}, {ptr = 0x0, field = 0,
        bytes = "\000\000\000\000\000\000\000"}}, bytes = '\000'
<repeats 15 times>}, sim_priv = {entries = {{ptr = 0xfffff820d9d8dd80,
field = 18446735418710351232,
        bytes = "\200\335\330\331 \370\377\377"}, {ptr = 0x0, field =
0, bytes = "\000\000\000\000\000\000\000"}}, bytes = "\200\335\330\331
\370\377\377\000\000\000\000\000\000\000"},
  qos = {etime = 0x0, sim_data = 0, periph_data = 1050626691830},
timeout = 0, softtimeout = {tv_sec = 0, tv_usec = 0}}
(kgdb) print *ccb_h->path
$2 = {periph = 0xdeadc0dedeadc0de, bus = 0xdeadc0dedeadc0de, target =
0xdeadc0dedeadc0de, device = 0xffffffff81a49810 <M_CAMPATH>}
(kgdb) print *ccb_h->path->bus
 access memory at address 0xdeadc0dedeadc0de

  Not sure how we ended up with device pointer appears to be valid
whilst the others are 0xdeadc0dedeadc0de.