xfs: fix xfs_bmap_validate_extent_raw when checking attr fork of rt files

The realtime flag only applies to the data fork, so don't use the
realtime block number checks on the attr fork of a realtime file.

Fixes: 30b0984d ("xfs: refactor bmap record validation")
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>