xfs: devirtualize ->leaf_hdr_to_disk

Replace the ->leaf_hdr_to_disk dir ops method with a directly called
xfs_dir_leaf_hdr_to_disk helper that takes care of the differences
between the v4 and v5 on-disk format.

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>