misc: use the checksum predicate function, not raw flag tests

misc: use the checksum predicate function, not raw flag tests

metadata_csum implies uninit_bg, and in fact forces the bit off for
rocompat with older implementations.  Therefore, to detect the
presence of checksums, we should use the predicate function to decide
if group descriptor checksums are turned on, not open-coded flag
tests.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>