virtio_net: zero is an invald queue_pairs number

Execute "ethtool -L eth0 combined 0" in guest, if multiqueue
is enabled, virtnet_send_command() will return -EINVAL error,
there is a validation in QEMU.

But if multiqueue is disabled, virtnet_set_queues() will just
return zero (success). We should return error for this situation.

Signed-off-by: Amos Kong <akong@redhat.com>
Acked-by:  Jason Wang <jasowang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/virtio_net.c

@@ -1285,7 +1285,7 @@ static int virtnet_set_channels(struct net_device *dev,
 	if (channels->rx_count || channels->tx_count || channels->other_count)
 		return -EINVAL;
 
-	if (queue_pairs > vi->max_queue_pairs)
+	if (queue_pairs > vi->max_queue_pairs || queue_pairs == 0)
 		return -EINVAL;
 
 	get_online_cpus();