xfs: fix endianness error when checking log block crc on big endian platforms

Since the checksum function and the field are both __le32, don't
perform endian conversion when comparing the two.  This fixes mount
failures on ppc64.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>