Updates to UFS/FFS superblock integrity checks when reading a superblock.

Check for an uninitialed (zero valued) fs_maxbsize and set it
to its minimum valid size (fs_bsize). Uninitialed fs_maxbsize
were left by older versions of makefs(8) and the superblock
integrity checks fail when they are found.

No legitimate superblocks should fail as a result of these changes.

MFC after: 1 week
Sponsored by: The FreeBSD Foundation