jbd2: gate checksum calculations on crc driver presence, not sb flags

Change the journal's checksum functions to gate on whether or not the
crc32c driver is loaded, and gate the loading on the superblock bits.
This prevents a journal crash if someone loads a journal in no-csum
mode and then randomizes the superblock, thus flipping on the feature
bits.

Tested-By: Nikolay Borisov <kernel@kyup.com>
Reported-by: Nikolay Borisov <kernel@kyup.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>