xfs: remove unused btree cursor bc_private.a.dfops field

The xfs_btree_cur.bc_private.a.dfops field is only ever initialized
by the refcountbt cursor init function. The only caller of that
function with a non-NULL dfops is from deferred completion context,
which already has attached to ->t_dfops.

In addition to that, the only actual reference of a.dfops is the
cursor duplication function, which means the field is effectively
unused.

Remove the dfops field from the bc_private.a union. Any future users
can acquire the dfops from the transaction. 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>

fs/xfs/scrub/common.c

@@ -499,7 +499,7 @@ xfs_scrub_ag_btcur_init(
 	/* Set up a refcountbt cursor for cross-referencing. */
 	if (sa->agf_bp && xfs_sb_version_hasreflink(&mp->m_sb)) {
 		sa->refc_cur = xfs_refcountbt_init_cursor(mp, sc->tp,
-				sa->agf_bp, agno, NULL);
+				sa->agf_bp, agno);
 		if (!sa->refc_cur)
 			goto err;
 	}