xfs: cleanup xfs_dir2_data_entsize

Remove the XFS_DIR2_DATA_ENTSIZE and XFS_DIR3_DATA_ENTSIZE and open
code them in their only caller, which now becomes so simple that
we can turn it into an inline function.

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>