ipvs: load balance ipv6 connections from a local process

This allows IPVS to load balance IPv6 connections made by a local process.
For example a proxy server running locally.

External client --> pound:443 -> Local:443 --> IPVS:80 --> RealServer

This is an extenstion to the IPv4 work done in this area
by Siim Põder and Malcolm Turnbull.

Cc: Siim Põder <siim@p6drad-teel.net>
Cc: Malcolm Turnbull <malcolm@loadbalancer.org>
Signed-off-by: Simon Horman <horms@verge.net.au>