[IP6TUNNEL]: Allow to create IP6 tunnels in net namespaces.

And no need in some IPPROTO_XXX enabling, since ipv6 code
doesn't have any filtering.

So, just set proper net and mark device with NETNS_LOCAL.

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