Clear all three journal checksum feature flags before turning on
whichever journal checksum options we want.  Rearrange the error
checking so that newer flags get complained about first.
Reported-by: TR Reardon <thomas_reardon@hotmail.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>