ipv6: Implement Any-IP support for IPv6.

ipv6: Implement Any-IP support for IPv6.

AnyIP is the capability to receive packets and establish incoming
connections on IPs we have not explicitly configured on the machine.

An example use case is to configure a machine to accept all incoming
traffic on eth0, and leave the policy of whether traffic for a given IP
should be delivered to the machine up to the load balancer.

Can be setup as follows:
  ip -6 rule from all iif eth0 lookup 200
  ip -6 route add local default dev lo table 200
(in this case for all IPv6 addresses)

Signed-off-by: Maciej Żenczykowski <maze@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>