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: NCong Wang <xiyou.wangcong@gmail.com>
Acked-by: NHannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>