"fsck -fB" panics: snapacct_ufs2: bad block

I noticed that my system hadn't freed up space in / after I accidently
filled it a few days ago.  "fsck -n" reported a number of unreferenced
files that lsof reported were not in use by any processes (I've reported
this problem previously).  My -current is about 6 weeks old but I don't
recall seeing any commits that might affect this.

I decided to see if I could get a background fsck to clean up the problem
without necessitating a reboot but "fsck -fB /" just panic'd:

panic: snapacct_ufs2: bad block
KDB: stack backtrace:
kdb_backtrace(c06e3fdb,c074b340,c06f4489,d74e7338,100) at kdb_backtrace+0x2e
panic(c06f4489,0,800,0,c32a0d80) at panic+0xb7
snapacct_ufs2(c509ab2c,c5aa95e0,c5aa9780,cdd43000,180c) at snapacct_ufs2+0x153
fullacct_ufs2(c509ab2c,c5aa9000,c5aa9780,cdd43000,180c) at fullacct_ufs2+0x44
indiracct_ufs2(c509ab2c,c46a2e38,0,9e68,0) at indiracct_ufs2+0x239
indiracct_ufs2(c509ab2c,c46a2e38,0,9de8,0) at indiracct_ufs2+0x32b
expunge_ufs2(c509ab2c,c52f0738,cdd43000,c0626fe0,1) at expunge_ufs2+0x54a
ffs_snapshot(c367f000,c4c844a0,d74e7940,6c,c3af5000) at ffs_snapshot+0x18dd
ffs_mount(c367f000,c3af5000,c06ecd82,2f9,d74e7a30) at ffs_mount+0xc91
vfs_domount(c3af5000,c36847c0,c3684800,1211300,c35acca0) at vfs_domount+0x5da
vfs_donmount(c3af5000,1211300,d74e7b98,c3ddd880,e) at vfs_donmount+0x522
kernel_mount(c48bc1e0,1211300,d74e7bdc,6c,805bb00) at kernel_mount+0x7e
ffs_cmount(c48bc1e0,bfbfeb30,1211300,c3af5000,c0736ac0) at ffs_cmount+0x84
mount(c3af5000,d74e7d04,c06ffc05,3ea,c37a5318) at mount+0x1d6
syscall(3b,3b,3b,0,200000) at syscall+0x2f3
Xint0x80_syscall() at Xint0x80_syscall+0x1f
--- syscall (21, FreeBSD ELF32, mount), eip = 0x280deed7, esp = 0xbfbfea3c, ebp = 0xbfbfebb8 ---
KDB: enter: panic

I haven't investigated yet but I've got a crash dump if anyone wants to
suggest places to look.

-- 
Peter Jeremy