With kernel sources as of yesterday I get the following:

panic: kernel diagnostic assertion "bytes != 0" failed: file 
"/usr/src/sys/miscfs/genfs/genfs_vnops.c", line 1327

Stopped in pid 1605 (sox) ...
db> tr
[...]
genfs_gop_write()
genfs_putpages()
VOP_PUTPAGES()
uvn_put()
ffs_full_fsync()
ffs_fsync()
VOP_FSYNC()
ffs_balloc()
VOP_BALLOC()
ffs_gop_alloc()
ufs_balloc_range()
ffs_write()
VOP_WRITE()
vn_write()
dofilewrite()
sys_write()

It is well possible that the disk is full or nearly full at this
point.
I see that there is a number of changes to ffs code from
yesterday to today... Anyway, I can reproduce the panic, and
if someone needs some details to debug, contact me soon. I'll
not touch the kernel and disk until tonight.

best regards
Matthias