sit: add net device refcount tracking to ip_tunnel

Note that other ip_tunnel users do not seem to hold a reference
on tunnel->dev. Probably needs some investigations.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

include/net/ip_tunnels.h

@@ -104,7 +104,10 @@ struct metadata_dst;
 struct ip_tunnel {
 	struct ip_tunnel __rcu	*next;
 	struct hlist_node hash_node;
+
 	struct net_device	*dev;
+	netdevice_tracker	dev_tracker;
+
 	struct net		*net;	/* netns for packet i/o */
 
 	unsigned long	err_time;	/* Time when the last ICMP error

net/ipv6/sit.c

@@ -521,7 +521,7 @@ static void ipip6_tunnel_uninit(struct net_device *dev)
 		ipip6_tunnel_del_prl(tunnel, NULL);
 	}
 	dst_cache_reset(&tunnel->dst_cache);
-	dev_put(dev);
+	dev_put_track(dev, &tunnel->dev_tracker);
 }
 
 static int ipip6_err(struct sk_buff *skb, u32 info)
@@ -1463,7 +1463,7 @@ static int ipip6_tunnel_init(struct net_device *dev)
 		dev->tstats = NULL;
 		return err;
 	}
-	dev_hold(dev);
+	dev_hold_track(dev, &tunnel->dev_tracker, GFP_KERNEL);
 	return 0;
 }