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>