Memory modified after free; most recently used by cred

Got this on a 12-processor machine under heavy load.  I'll have to try
and reproduce it with memguard (currently I have memguard looking for
another use-after-free in the softupdates code).

Kris

Memory modified after free 0xfffff80015c58e00(248) val=fffff800 _at_ 0xfffff80015c58e38
panic: Most recently used by cred

cpuid = 6
KDB: enter: panic
[thread pid 13926 tid 100573 ]
Stopped at      kdb_enter+0x3c: ta              %xcc, 1
db> wh
Tracing pid 13926 tid 100573 td 0xfffff800680e5560
panic() at panic+0x16c
mtrash_ctor() at mtrash_ctor+0x6c
uma_zalloc_arg() at uma_zalloc_arg+0x110
malloc() at malloc+0x114
crget() at crget+0x10
crdup() at crdup+0x4
kern_access() at kern_access+0xc
access() at access+0x10
syscall() at syscall+0x2d4