[IPV6]: Use ctl paths to register addrconf sysctls

This looks very much like the patch for ipv4's devinet.

This is also intended to help us with the net namespaces
and saves the ipv6.ko size by ~320 bytes.

The difference from the first version is just the patch
offsets, that changed due to changes in the patch #2.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>