ipv6: AF_INET6 link address family

IPv6 already exposes some address family data via netlink in the
IFLA_PROTINFO attribute if RTM_GETLINK request is sent with the
address family set to AF_INET6. We take over this format and
reuse all the code.

Signed-off-by: Thomas Graf <tgraf@infradead.org>
Cc: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>