virtio-net: fix the race between channels setting and refill

Commit 55257d72 (virtio-net: fill only rx queues
which are being used) tries to refill on demand when changing the number of
channels by call try_refill_recv() directly, this may race:

- the refill work who may do the refill in the same time
- the try_refill_recv() called in bh since napi was not disabled

Which may led guest complain during setting channels:

virtio_net virtio0: input.1:id 0 is not a head!

Solve this issue by scheduling a refill work which can guarantee the
serialization of refill.

Cc: Sasha Levin <sasha.levin@oracle.com>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

drivers/net/virtio_net.c

@@ -900,7 +900,6 @@ static int virtnet_set_queues(struct virtnet_info *vi, u16 queue_pairs)
 	struct scatterlist sg;
 	struct virtio_net_ctrl_mq s;
 	struct net_device *dev = vi->dev;
-	int i;
 
 	if (!vi->has_cvq || !virtio_has_feature(vi->vdev, VIRTIO_NET_F_MQ))
 		return 0;
@@ -914,10 +913,8 @@ static int virtnet_set_queues(struct virtnet_info *vi, u16 queue_pairs)
 			 queue_pairs);
 		return -EINVAL;
 	} else {
-		for (i = vi->curr_queue_pairs; i < queue_pairs; i++)
-			if (!try_fill_recv(&vi->rq[i], GFP_KERNEL))
-				schedule_delayed_work(&vi->refill, 0);
 		vi->curr_queue_pairs = queue_pairs;
+		schedule_delayed_work(&vi->refill, 0);
 	}
 
 	return 0;