virtio_net: Fix recursive call to cpus_read_lock()

virtnet_set_channels can recursively call cpus_read_lock if CONFIG_XPS
and CONFIG_HOTPLUG are enabled.

The path is:
    virtnet_set_channels - calls get_online_cpus(), which is a trivial
wrapper around cpus_read_lock()
    netif_set_real_num_tx_queues
    netif_reset_xps_queues_gt
    netif_reset_xps_queues - calls cpus_read_lock()

This call chain and potential deadlock happens when the number of TX
queues is reduced.

This commit the removes netif_set_real_num_[tr]x_queues calls from
inside the get/put_online_cpus section, as they don't require that it
be held.

Fixes: 47be2479 ("virtio-net: fix the set affinity bug when CPU IDs are not consecutive")
Signed-off-by: Jeff Dike <jdike@akamai.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Link: https://lore.kernel.org/r/20201223025421.671-1-jdike@akamai.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/virtio_net.c

@@ -2093,14 +2093,16 @@ static int virtnet_set_channels(struct net_device *dev,
 
 	get_online_cpus();
 	err = _virtnet_set_queues(vi, queue_pairs);
-	if (!err) {
-		netif_set_real_num_tx_queues(dev, queue_pairs);
-		netif_set_real_num_rx_queues(dev, queue_pairs);
-
-		virtnet_set_affinity(vi);
+	if (err) {
+		put_online_cpus();
+		goto err;
 	}
+	virtnet_set_affinity(vi);
 	put_online_cpus();
 
+	netif_set_real_num_tx_queues(dev, queue_pairs);
+	netif_set_real_num_rx_queues(dev, queue_pairs);
+ err:
 	return err;
 }