xfs: remove struct xfs_bmalloca dfops field

Now that bma.dfops is only assigned from ->t_dfops, replace all
accesses to the former with the latter and remove the unnecessary
field. This patch does not change behavior.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-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>