"panic: malloc(M_WAITOK) in non-sleepable context" after r364296 -> r364341

Just did a src update from r364296 -> r364341 on amd64 on my bild
machine (laptop is still working -- it built firefox earlier, which
takes some time).  On initial reboot, the build machine panicked:

umass0:  SCSI over Bulk-Only; quirks = 0x4000
umass0:6:0: Attached to scbus6
(probe0:umass-sim0:0:0:0): Down reving Protocol Version from 2 to 0?
panic: malloc(M_WAITOK) in non-sleepable context
cpuid = 1
time = 20
KDB: stack backtrace:
db_trace_self_wrapper() at db_trace_self_wrapper+0x2b/frame 0xffffffff8218d440
vpanic() at vpanic+0x182/frame 0xffffffff8218d490
panic() at panic+0x43/frame 0xffffffff8218d4f0
malloc() at malloc+0x20c/frame 0xffffffff8218d540
devfs_alloc() at devfs_alloc+0x28/frame 0xffffffff8218d570
make_dev_sv() at make_dev_sv+0x97/frame 0xffffffff8218d600
make_dev_s() at make_dev_s+0x3b/frame 0xffffffff8218d660
passregister() at passregister+0x3e7/frame 0xffffffff8218d8b0
cam_periph_alloc() at cam_periph_alloc+0x57b/frame 0xffffffff8218d980
passasync() at passasync+0x5c/frame 0xffffffff8218d9c0
xpt_async_process_dev() at xpt_async_process_dev+0x152/frame 0xffffffff8218da10
xpt_async_process() at xpt_async_process+0x334/frame 0xffffffff8218db20
xpt_done_process() at xpt_done_process+0x3a3/frame 0xffffffff8218db60
xpt_done_td() at xpt_done_td+0xf5/frame 0xffffffff8218dbb0
fork_exit() at fork_exit+0x80/frame 0xffffffff8218dbf0
fork_trampoline() at fork_trampoline+0xe/frame 0xffffffff8218dbf0
--- trap 0, rip = 0, rsp = 0, rbp = 0 ---
KDB: enter: panic
[ thread pid 16 tid 100068 ]
Stopped at      kdb_enter+0x37: movq    $0,0x10b70f6(%rip)
db> 


So -- it's at the debugger prompt; given sufficient hints, I'm happy
to poke at it & report back.

Yesterday's (r364296) verbose dmesg.boot is at
http://www.catwhisker.org/~david/FreeBSD/history/freebeast.13_dmesg.txt
(in case that's useful).

Peace,
david
-- 
David H. Wolfskill				david_at_catwhisker.org
Donald Trump is either ignorant of how to govern or is refusing to do so.

See http://www.catwhisker.org/~david/publickey.gpg for my public key.