_mtx_assert() panic

One of the package machines died with this, running -current from a
few days ago.

Fatal trap 12: page fault while in kernel mode
fault virtual address   = 0x1c
fault code              = supervisor read, page not present
instruction pointer     = 0x8:0xc055ea8e
stack pointer           = 0x10:0xd9c029c8
frame pointer           = 0x10:0xd9c029e8
code segment            = base 0x0, limit 0xfffff, type 0x1b
                        = DPL 0, pres 1, def32 1, gran 1
processor eflags        = interrupt enabled, resume, IOPL = 0
current process         = 6332 (pkg_create)
kernel: type 12 trap, code=0
Stopped at      _mtx_assert+0x4e:       movl    0x1c(%ebx),%eax
db> trace
_mtx_assert(0,1,c073fd34,dd2,ff) at _mtx_assert+0x4e
vfs_bio_clrbuf(ceb9dca0,2,0,d9c02a38,ceb9dca0) at vfs_bio_clrbuf+0x1fd
ufs_strategy(d9c02b08,d9c02b34,c05b6967,d9c02b08,0) at ufs_strategy+0xce
ufs_vnoperate(d9c02b08,0,0,8000,0) at ufs_vnoperate+0x18
cluster_read(c5c4cdb0,1ce5df0,0,2,0) at cluster_read+0x487
ffs_read(d9c02be0,1020002,c5081e40,1ff,d9c02c7c) at ffs_read+0x2f0
vn_read(c61ae770,d9c02c7c,c53a4880,0,c5081e40) at vn_read+0x1a2
dofileread(c5081e40,c61ae770,5,bfbfe4a0,400) at dofileread+0xd9
read(c5081e40,d9c02d10,c0755a1e,3ed,3) at read+0x6b
syscall(2f,1002f,bfbf002f,0,1cd5df0) at syscall+0x2c0
Xint0x80_syscall() at Xint0x80_syscall+0x1d
--- syscall (3, FreeBSD ELF32, read), eip = 0x28210b6f, esp = 0xbfbfe39c, ebp = 0xbfbfe8b8 ---
db>