[IPV6]: Use sysctl paths to register ipv6 sysctl tables

I have already done this for core, ipv4 and tr tables, so repeat this
for the ipv6 ones.

This makes the ipv6.ko smaller and creates the ground needed for net
namespaces support in ipv6.ko ssctls.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>