net: loopback: Hold rtnl_net_lock() in blackhole_netdev_init().

blackhole_netdev is the global device in init_net.

Let's hold rtnl_net_lock(&init_net) in blackhole_netdev_init().

While at it, the unnecessary dev_net_set() call is removed, which
is done in alloc_netdev_mqs().

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://patch.msgid.link/20250114081352.47404-1-kuniyu@amazon.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/loopback.c

@@ -264,13 +264,12 @@ static int __init blackhole_netdev_init(void)
 	if (!blackhole_netdev)
 		return -ENOMEM;
 
-	rtnl_lock();
+	rtnl_net_lock(&init_net);
 	dev_init_scheduler(blackhole_netdev);
 	dev_activate(blackhole_netdev);
-	rtnl_unlock();
+	rtnl_net_unlock(&init_net);
 
 	blackhole_netdev->flags |= IFF_UP | IFF_RUNNING;
-	dev_net_set(blackhole_netdev, &init_net);
 
 	return 0;
 }