virtio-net: check feature before configuring the vq coalescing command

Virtio spec says:

	The driver MUST have negotiated the VIRTIO_NET_F_VQ_NOTF_COAL
	feature when issuing commands VIRTIO_NET_CTRL_NOTF_COAL_VQ_SET
	and VIRTIO_NET_CTRL_NOTF_COAL_VQ_GET.

So we add the feature negotiation check to
virtnet_send_{r,t}x_ctrl_coal_vq_cmd as a basis for the next bugfix patch.

Suggested-by: Michael S. Tsirkin <[email protected]>
Signed-off-by: Heng Qi <[email protected]>
Acked-by: Jason Wang <[email protected]>
Signed-off-by: David S. Miller <[email protected]>

drivers/net/virtio_net.c

@@ -3658,6 +3658,9 @@ static int virtnet_send_rx_ctrl_coal_vq_cmd(struct virtnet_info *vi,
 {
 	int err;
 
+	if (!virtio_has_feature(vi->vdev, VIRTIO_NET_F_VQ_NOTF_COAL))
+		return -EOPNOTSUPP;
+
 	err = virtnet_send_ctrl_coal_vq_cmd(vi, rxq2vq(queue),
 					    max_usecs, max_packets);
 	if (err)
@@ -3675,6 +3678,9 @@ static int virtnet_send_tx_ctrl_coal_vq_cmd(struct virtnet_info *vi,
 {
 	int err;
 
+	if (!virtio_has_feature(vi->vdev, VIRTIO_NET_F_VQ_NOTF_COAL))
+		return -EOPNOTSUPP;
+
 	err = virtnet_send_ctrl_coal_vq_cmd(vi, txq2vq(queue),
 					    max_usecs, max_packets);
 	if (err)