[IPIP]: Use proper net in (mostly) routing calls.

There are some ip_route_output_key() calls in there that require
a proper net so give one to them.

Besides - give a proper net to a single __get_dev_by_index call
in ipip_tunnel_bind_dev().

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