recursed on non-recursive lock (sleep mutex) vnode interlock _at_ /var/portbuild/sparc64/src-client/sys/ufs/ufs/ufs_ihash.c:128

One of my sparc64 package machines (running -current from Nov 21) died
overnight with the following:

recursed on non-recursive lock (sleep mutex) vnode interlock _at_ /var/portbuild/sparc64/src-client/sys/ufs/ufs/ufs_ihash.c:128
first acquired _at_ /var/portbuild/sparc64/src-client/sys/ufs/ufs/ufs_ihash.c:128
panic: recurse
cpuid = 0;
Debugger("panic")
Stopped at      Debugger+0x1c:  ta              %xcc, 1
db> trace
panic() at panic+0x174
witness_lock() at witness_lock+0x3b4
_mtx_lock_flags() at _mtx_lock_flags+0x9c
ufs_ihashget() at ufs_ihashget+0x94
ffs_vget() at ffs_vget+0x20
ufs_lookup() at ufs_lookup+0xb2c
ufs_vnoperate() at ufs_vnoperate+0x1c
vfs_cache_lookup() at vfs_cache_lookup+0x330
ufs_vnoperate() at ufs_vnoperate+0x1c
lookup() at lookup+0x408
namei() at namei+0x254
vn_open_cred() at vn_open_cred+0x208
vn_open() at vn_open+0x18
kern_open() at kern_open+0x84
open() at open+0x14
syscall() at syscall+0x308
-- syscall (5, FreeBSD ELF64, open) %o7=0x4038c2b0 --
userland() at 0x40395948
user trace: trap %o7=0x4038c2b0
pc 0x40395948, sp 0x7fdffffdaf1
pc 0x4038b47c, sp 0x7fdffffdc31
pc 0x101778, sp 0x7fdffffdcf1
pc 0x101378, sp 0x7fdffffddb1
pc 0x100f80, sp 0x7fdffffde71
pc 0x4020a234, sp 0x7fdffffdf31
done