Verify the (ext4) superblock checksum of an external journal device
and prompt to correct the checksum if nothing else is wrong with the
superblock.
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Cc: TR Reardon <thomas_reardon@hotmail.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>