net: correct error path in rtnl_newlink()

I saw the following BUG when ->newlink() fails in rtnl_newlink():

[   40.240058] kernel BUG at net/core/dev.c:6438!

this is due to free_netdev() is not supposed to be called before
netdev is completely unregistered, therefore it is not correct
to call free_netdev() here, at least for ops->newlink!=NULL case,
many drivers call it in ->destructor so that rtnl_unlock() will
take care of it, we probably don't need to do anything here.

Cc: David S. Miller <davem@davemloft.net>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: Cong Wang <cwang@twopensource.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/rtnetlink.c

@@ -1963,16 +1963,21 @@ static int rtnl_newlink(struct sk_buff *skb, struct nlmsghdr *nlh)
 
 		dev->ifindex = ifm->ifi_index;
 
-		if (ops->newlink)
+		if (ops->newlink) {
 			err = ops->newlink(net, dev, tb, data);
-		else
+			/* Drivers should call free_netdev() in ->destructor
+			 * and unregister it on failure so that device could be
+			 * finally freed in rtnl_unlock.
+			 */
+			if (err < 0)
+				goto out;
+		} else {
 			err = register_netdevice(dev);
-
-		if (err < 0) {
-			free_netdev(dev);
-			goto out;
+			if (err < 0) {
+				free_netdev(dev);
+				goto out;
+			}
 		}
-
 		err = rtnl_configure_link(dev, ifm);
 		if (err < 0)
 			unregister_netdevice(dev);