sit: proper dev_{hold|put} in ndo_[un]init methods

After adopting CONFIG_PCPU_DEV_REFCNT=n option, syzbot was able to trigger
a warning [1]

Issue here is that:

- all dev_put() should be paired with a corresponding prior dev_hold().

- A driver doing a dev_put() in its ndo_uninit() MUST also
  do a dev_hold() in its ndo_init(), only when ndo_init()
  is returning 0.

Otherwise, register_netdevice() would call ndo_uninit()
in its error path and release a refcount too soon.

Fixes: 919067cc84 ("net: add CONFIG_PCPU_DEV_REFCNT")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: syzbot <syzkaller@googlegroups.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/sit.c

@@ -218,8 +218,6 @@ static int ipip6_tunnel_create(struct net_device *dev)
 
 	ipip6_tunnel_clone_6rd(dev, sitn);
 
-	dev_hold(dev);
-
 	ipip6_tunnel_link(sitn, t);
 	return 0;
 
@@ -1456,7 +1454,7 @@ static int ipip6_tunnel_init(struct net_device *dev)
 		dev->tstats = NULL;
 		return err;
 	}
-
+	dev_hold(dev);
 	return 0;
 }