ipv6: fix nl group when advertising a new link

New idev are advertised with NL group RTNLGRP_IPV6_IFADDR, but
should use RTNLGRP_IPV6_IFINFO.
Bug was introduced by commit 8d7a76c9.

Signed-off-by: Wang Xuefu <xuefu.wang@6wind.com>
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Acked-by: Thomas Graf <tgraf@infradead.org>
Signed-off-by: David S. Miller <davem@davemloft.net>