ufs/devfs "lock order reversal" on poweroff

Hi

On r278909 (and probably earlier) I get the following when I run
"poweroff" (retyped from a video of it I had to record, since it
disappears very quickly):

Syncing disks, vnodes remaining...4 1 0 0 done
All buffers synced.
lock order reversal:
 1st 0xfffff80014d4d060 ufs (ufs) 0 /usr/src/sys/kern/vfs_mount.c:1229
 2nd 0xfffff00014a695f0 devfs (devfs) 0 /usr/src/sys/kern/vfs_subr.c:2176
KDB: stack backtrace:
db_trace_self_wrapper() at db_trace_self_wrapper+0x2b/frame ...
witness_checkorder() at witness_checkorder+...
__lockmgr_args() at __lockmgr_args+...
vop_stdlock() at vop_stdlock+0x3c/frame ..
VOP_LOOCK1_AVP()
_vm_lock()
vget()
devfs_allocv()
devfs_root()
dounmount()
vfs_unmountall()
kern_reboot()
sys_reboot()
amd64_syscall()
Xfast_syscall()
--- syscall (55, FreeBSD ELF64, sys_reboot), ript = 0x40fd1c, rsp =
..., rbp= ...
Uptime: ...

Thank you
Damjan