[GRE]: Use proper net in routing calls.

As for the IPIP tunnel, there are some ip_route_output_key()
calls in there that require a proper net so give one to them.

And a proper net for the __get_dev_by_index hanging around.

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