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

Have to add a check that the computed cylinder group size does not
exceed the block size of the filesystem.

It is also necessary to validate additional parameters when a
superblock is going to be used in read-only mode if its supplementary
information is going to be read in to ensure that the size and
location of the supplementary information is valid. Also when a
warning is raised let it be accepted, but bound the flagged field
to the value checked by the warning.

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

Reported by: Bob Prohaska, John-Mark Gurney, and Mark Millard
Tested by: Peter Holm
Reviewed by: Peter Holm
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D38668