xfs: move the node header size to struct xfs_da_geometry

Move the node header size field to struct xfs_da_geometry, and remove
the now unused non-directory dir ops infrastructure.

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>