Old FFS triggers assertion in BUFRD()

[Valery Ushakov <uwe%stderr.spb.ru@localhost>]

I have found OpenWindows Version 3 CD in a drawer.  The label claims
"ISO 9660 format", but it's really an FFS image.

I was able to mount it with a little tweak - ffs_superblock_validate()
checked only fs_size, but this CD from 1991 only has fs_old_size (fix
committed).

The next hiccup I ran into was an assertion in ufs_readwrite.c:172
(rump is compiled with DIAGNOSTIC).

    KASSERT(vp->v_type != VLNK || ump->um_maxsymlinklen != 0 ||
        DIP(ip, blocks) == 0);

It was triggered by di_blocks being 2.

May be someone with enough FFS clue could take a look?  Disc image
available upon request.

-uwe