xfs: devirtualize ->data_entsize

Replace the ->data_entsize dir ops method with a directly called
xfs_dir2_data_entsize helper that takes care of the differences between
the directory format with and without the file type field.

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>