can: gw: Properly fill the netlink header when responding to RTM_GETROUTE

 - set message type to RTM_NEWROUTE
 - relate to original request by inheriting the sequence and port number.
 - set NLM_F_MULTI because it's a dump and more messages will follow

Signed-off-by: Thomas Graf <tgraf@suug.ch>
Tested-by: Oliver Hartkopp <socketcan@hartkopp.net>
Acked-by: Oliver Hartkopp <socketcan@hartkopp.net>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>