xfs: add missing rmap error return

xfs_rmap_lookup_le_range can return errors, so we need to check for
them and bail out.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Brian Foster <bfoster@redhat.com>