virtio-net: on tx, only call napi_disable if tx napi is on

As of tx napi, device down (`ip link set dev $dev down`) hangs unless
tx napi is enabled. Else napi_enable is not called, so napi_disable
will spin on test_and_set_bit NAPI_STATE_SCHED.

Only call napi_disable if tx napi is enabled.

Fixes: 5a719c2552ca ("virtio-net: transmit napi")
Reported-by: NJason Wang <jasowang@redhat.com>
Signed-off-by: NWillem de Bruijn <willemb@google.com>
Acked-by: NMichael S. Tsirkin <mst@redhat.com>
Acked-by: NJason Wang <jasowang@redhat.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/virtio_net.c

@@ -997,6 +997,12 @@ static void virtnet_napi_tx_enable(struct virtnet_info *vi,
 	return virtnet_napi_enable(vq, napi);
 }
 
+static void virtnet_napi_tx_disable(struct napi_struct *napi)
+{
+	if (napi->weight)
+		napi_disable(napi);
+}
+
 static void refill_work(struct work_struct *work)
 {
 	struct virtnet_info *vi =
@@ -1445,7 +1451,7 @@ static int virtnet_close(struct net_device *dev)
 
 	for (i = 0; i < vi->max_queue_pairs; i++) {
 		napi_disable(&vi->rq[i].napi);
-		napi_disable(&vi->sq[i].napi);
+		virtnet_napi_tx_disable(&vi->sq[i].napi);
 	}
 
 	return 0;
@@ -1803,7 +1809,7 @@ static void virtnet_freeze_down(struct virtio_device *vdev)
 	if (netif_running(vi->dev)) {
 		for (i = 0; i < vi->max_queue_pairs; i++) {
 			napi_disable(&vi->rq[i].napi);
-			napi_disable(&vi->sq[i].napi);
+			virtnet_napi_tx_disable(&vi->sq[i].napi);
 		}
 	}
 }