r366623: lock order reversal in ZFS

On a recent CURRENT bos acting as poudriere host, we face after a crash
under heavy load and extensive swap usage now this lock order reversal
warning on the console:

lock order reversal:
 1st 0xfffff8007bdd9e00 zfs (zfs, lockmgr) _at_
/usr/src/sys/kern/vfs_mount.c:1018 2nd 0xfffff8034abfc070 devfs (devfs,
lockmgr) _at_ /usr/src/sys/kern/vfs_mount.c:1029 lock order zfs -> devfs
attempted at: #0 0xffffffff80f3380d at witness_checkorder+0xebd
#1 0xffffffff80e8ba4b at lockmgr_xlock+0xab
#2 0xffffffff80fc3e04 at _vn_lock+0x54
#3 0xffffffff80fa0e34 at vfs_domount+0xde4
#4 0xffffffff80f9f598 at vfs_donmount+0x898
#5 0xffffffff80f9ecb9 at sys_nmount+0x69
#6 0xffffffff81461255 at amd64_syscall+0x135
#7 0xffffffff814335ee at fast_syscall_common+0xf8

System: FreeBSD 13.0-CURRENT #71 r366623: Sun Oct 11 08:59:17 CEST 2020