xfs: always pass a valid hdr to xfs_dir3_leaf_check_int

Move the code for extracting the incore header to the only caller that
didn't already do that.

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