[IP6TUNNEL]: Use proper net instead of init_net stubs.

All the ip_route_output_key(), dev_get_by_...() and ipv6_chk_addr()
calls are now stubbed with init_net.

Fortunately, all the places already have where to get the proper
net from.

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