NetBSD can actually make some sense of this FreeBSD filesystem though:
# fsck -n /dev/mapper/rscratch-fbsd--test.0
** /dev/mapper/rscratch-fbsd--test.0 (NO WRITE)
Invalid quota magic number
CONTINUE? yes
** File system is already clean
** Last Mounted on /mnt
** Phase 1 - Check Blocks and Sizes
** Phase 2 - Check Pathnames
** Phase 3 - Check Connectivity
** Phase 4 - Check Reference Counts
** Phase 5 - Check Cyl groups
SUMMARY INFORMATION BAD
SALVAGE? no
BLK(S) MISSING IN BIT MAPS
SALVAGE? no
** Phase 6 - Check Quotas
CLEAR SUPERBLOCK QUOTA FLAG? no
2 files, 2 used, 7612693 free (21 frags, 951584 blocks, 0.0% fragmentation)
***** UNRESOLVED INCONSISTENCIES REMAIN *****
I'm not sure if those problems are to be expected with a FreeBSD-created
filesystem or not. Probably the "Invalid quota magic number" is normal,
but I'm not sure about the "BLK(s) MISSING IN BIT MAPS". Have FreeBSD
and NetBSD FFS diverged this much? I won't try to mount it, especially
not from the dom0.