panic: ffs_reallocblk: start == end

[Kris Kennaway]

I got this panic on an 8 core amd64 running 8.0 when writing to a ufs on 
a md device:

panic: ffs_reallocblk: start == end

db> wh
Tracing pid 59911 tid 100115 td 0xffffff0003b90000
kdb_enter() at kdb_enter+0x31
panic() at panic+0x1c0
ffs_reallocblks() at ffs_reallocblks+0xb11
VOP_REALLOCBLKS_APV() at VOP_REALLOCBLKS_APV+0xb9
cluster_write() at cluster_write+0x38a
ffs_write() at ffs_write+0x575
VOP_WRITE_APV() at VOP_WRITE_APV+0x147
vn_write() at vn_write+0x213
dofilewrite() at dofilewrite+0x9a
kern_writev() at kern_writev+0x4f
write() at write+0x4b
syscall() at syscall+0x301
Xfast_syscall() at Xfast_syscall+0xab
--- syscall (4, FreeBSD ELF64, write), rip = 0x800a65d0c, rsp = 
0x7fffffffd268, rbp = 0x2800 ---

Kris