xfs: move the max dir2 free bests count to struct xfs_da_geometry

Move the max free bests count towards our structure for dir/attr
geometry parameters.

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>