virtio-vdpa: don't set callback if virtio doesn't need it

mainline inclusion
from mainline-v5.19-rc1
commit ea239a67
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I5WXCZ
CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=ea239a67461a34839e2172ead3043295a9173389

----------------------------------------------------------------------

There's no need for setting callbacks for the driver that doesn't care
about that.
Signed-off-by: NJason Wang <jasowang@redhat.com>
Signed-off-by: NGautam Dawar <gdawar@xilinx.com>
Message-Id: <20220330180436.24644-3-gdawar@xilinx.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NPengyuan Zhao <zhaopengyuan@hisilicon.com>

drivers/virtio/virtio_vdpa.c

@@ -184,7 +184,7 @@ virtio_vdpa_setup_vq(struct virtio_device *vdev, unsigned int index,
 	}
 
 	/* Setup virtqueue callback */
-	cb.callback = virtio_vdpa_virtqueue_cb;
+	cb.callback = callback ? virtio_vdpa_virtqueue_cb : NULL;
 	cb.private = info;
 	ops->set_vq_cb(vdpa, index, &cb);
 	ops->set_vq_num(vdpa, index, virtqueue_get_vring_size(vq));