ext4: validate external journal superblock checksum

If the external journal device has metadata_csum enabled, verify
that the superblock checksum matches the block before we try to
mount.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>