xfs: move the dir2 free header size to struct xfs_da_geometry

Move the free header size 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>