ipv6: exit early in addrconf_notify() if IPv6 is disabled

If IPv6 is explicitly disabled before the interface comes up,
it makes no sense to continue when it comes up, even just
print a message.

(I am not sure about other cases though, so I prefer not to touch)
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/addrconf.c

@@ -2844,6 +2844,9 @@ static int addrconf_notify(struct notifier_block *this, unsigned long event,
 		if (dev->flags & IFF_SLAVE)
 			break;
 
+		if (idev && idev->cnf.disable_ipv6)
+			break;
+
 		if (event == NETDEV_UP) {
 			if (!addrconf_qdisc_ok(dev)) {
 				/* device is not ready yet. */