[NETNS][IPV6]: Make a subsystem for af_inet6.

This patch add a network namespace subsystem for the af_inet6 module.
It does nothing right now, but one of its purpose is to receive the
different variables for sysctl in order to initialize them.

When the sysctl variable will be moved to the network namespace
structure, they will be no longer initialized as global static
variables, so we must find a place to initialize them. Because the
sysctl can be disabled, it has no sense to store them in the
sysctl_net_ipv6 file.

Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>