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.

Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D38668

(cherry picked from commit 0eabe33087d8cb69ae800daae322f12a8f16bd10)