Panic on shutdown, r243674M

Rev 243674 with some minor local changes.

#12 0xffffffff80bfd775 in trap_fatal (frame=0xffffff800025e870,
    eva=<value optimized out>)
    at /home/emaste/src/head-ro/sys/amd64/amd64/trap.c:867
#13 0xffffffff80bfda3a in trap_pfault (frame=0x0, usermode=0)
    at /home/emaste/src/head-ro/sys/amd64/amd64/trap.c:698
#14 0xffffffff80bfd226 in trap (frame=0xffffff800025e870)
    at /home/emaste/src/head-ro/sys/amd64/amd64/trap.c:463
#15 0xffffffff80be71a2 in calltrap () at /tmp/exception-N6d6io.s:179
#16 0xffffffff808bb9c0 in rman_set_bustag ()
    at /home/emaste/src/head-ro/sys/kern/subr_rman.c:941
#17 0xffffffff80356c9d in acpi_cpu_idle ()
    at /home/emaste/src/head-ro/sys/dev/acpica/acpi_cpu.c:1020
#18 0xffffffff80beb268 in cpu_idle_acpi (busy=0)
    at /home/emaste/src/head-ro/sys/amd64/amd64/machdep.c:685
#19 0xffffffff80beb308 in cpu_idle (busy=0)
    at /home/emaste/src/head-ro/sys/amd64/amd64/machdep.c:839
#20 0xffffffff808a6058 in sched_idletd (dummy=<value optimized out>)
    at /home/emaste/src/head-ro/sys/kern/sched_ule.c:2665
#21 0xffffffff808524d4 in fork_exit (
    callout=0xffffffff808a5e20 <sched_idletd>, arg=0x0,
    frame=0xffffff800025eac0)
    at /home/emaste/src/head-ro/sys/kern/kern_fork.c:995
#22 0xffffffff80be76de in fork_trampoline () at /tmp/exception-N6d6io.s:501
#23 0x0000000000000000 in ?? ()