net: reinstate rtnl in call_netdevice_notifiers()

Eric Biederman pointed out that not holding RTNL while calling
call_netdevice_notifiers() was racy.

This patch is a direct transcription his feedback
against commit 0115e8e30d6fc (net: remove delay at device dismantle)

Thanks Eric !

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Tom Herbert <therbert@google.com>
Cc: Mahesh Bandewar <maheshb@google.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Gao feng <gaofeng@cn.fujitsu.com>
Acked-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv6/addrconf.c b/net/ipv6/addrconf.c
index e581009..6bc85f7 100644
--- a/net/ipv6/addrconf.c
+++ b/net/ipv6/addrconf.c
@@ -2566,14 +2566,10 @@
 			   void *data)
 {
 	struct net_device *dev = (struct net_device *) data;
-	struct inet6_dev *idev;
+	struct inet6_dev *idev = __in6_dev_get(dev);
 	int run_pending = 0;
 	int err;
 
-	if (event == NETDEV_UNREGISTER_FINAL)
-		return NOTIFY_DONE;
-
-	idev = __in6_dev_get(dev);
 	switch (event) {
 	case NETDEV_REGISTER:
 		if (!idev && dev->mtu >= IPV6_MIN_MTU) {