can: gw: make routing to the incoming CAN interface configurable

Introduce new configuration flag CGW_FLAGS_CAN_IIF_TX_OK to configure if a
CAN sk_buff that has been routed with can-gw is allowed to be send back to
the originating CAN interface.

Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>