xfs: log proper length of superblock

xfs_trans_log_buf takes first byte, last byte as args.  In this
case, it should be from 0 to sizeof() - 1.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>