xfs: devirtualize ->data_bestfree_p

Replace the ->data_bestfree_p dir ops method with a directly called
xfs_dir2_data_bestfree_p 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>