[NETNS][IPV6]: Make the ipv6 sysctl to be a netns subsystem.

The initialization of the sysctl for the ipv6 protocol is changed to a
network namespace subsystem. That means when a new network namespace
is created the initialization function for the sysctl will be called.

That do not change the behavior of the sysctl in case of the kernel
with the network namespace disabled.

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