libext2fs: block group checksum should use metadata_csum algorithm

libext2fs: block group checksum should use metadata_csum algorithm

Change the block group algorithm to use the same algorithm as the rest
of the metadata_csum.  This mostly involves providing a helper
function to tell if group descriptors should have checksums set or
verified, and modifying the gdt checksum code to use the correct
algorithm.

Signed-off-by: Darrick J. Wong <djwong@us.ibm.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>