ipv6: notify userspace when we added or changed an ipv6 token

NetworkManager might want to know that it changed when the router advertisement
arrives.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Cc: Hannes Frederic Sowa <hannes@stressinduktion.org>
Cc: Daniel Borkmann <dborkman@redhat.com>
Acked-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>