[NETNS]: Create ipv6 devconf-s for namespaces

This is the core. Declare and register the pernet subsys for
addrconf. The init callback the will create the devconf-s.

The init_net will reuse the existing statically declared confs,
so that accessing them from inside the ipv6 code will still
work.

The register_pernet_subsys() is moved above the ipv6_add_dev()
call for loopback, because this function will need the
net->devconf_dflt pointer to be already set.

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