vfs_hash_get panic

I'm running amd64 SMP -current around Dec 14. It seems that I can reproduce
this panic by 'mergemaster -iU' at /usr/src reliably.

Fatal trap 12: page fault while in kernel mode
cpuid = 0; apic id = 00
fault virtual address   = 0x68
fault code              = supervisor read data, page not present
instruction pointer     = 0x8:0xffffffff802e0ce3
stack pointer           = 0x10:0xffffffffa0019510
frame pointer           = 0x10:0xffffffffa0019570
code segment            = base 0x0, limit 0xfffff, type 0x1b
                        = DPL 0, pres 1, long 1, def32 0, gran 1
processor eflags        = interrupt enabled, resume, IOPL = 0
current process         = 786 (mtree)

Due to no remote console attached, the console ddb backtrace
is available at flickr:

http://flickr.com/photos/rafan/323030710/

I put my kernel conf and kgdb output here:

http://www.rafan.org/FreeBSD/panic/vfs_hash_get/KERNEL
http://www.rafan.org/FreeBSD/panic/vfs_hash_get/kgdb.txt

I saw there is another report on _at_hackers this Oct. But it seems
the originator solves the problem by upgrading to latest RELENG_6.

Any suggestions? I can provide more information if someone interested.

Regards,
Rong-En Fan