virtio_net: replace vq2rxq with vq2txq where appropriate

While not harmful, using vq2rxq where it's always sq appears odd.
Replace it with the more appropriate vq2txq for clarity and correctness.

Fixes: 89f8667 ("virtio_net: xsk: tx: support xmit xsk buffer")
Signed-off-by: Koichiro Den <koichiro.den@canonical.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Author: lkpdn

drivers/net/virtio_net.c

@@ -6213,7 +6213,7 @@ static void virtnet_sq_free_unused_buf(struct virtqueue *vq, void *buf)
 {
 	struct virtnet_info *vi = vq->vdev->priv;
 	struct send_queue *sq;
-	int i = vq2rxq(vq);
+	int i = vq2txq(vq);
 
 	sq = &vi->sq[i];