can: network namespace support for CAN gateway

The CAN gateway was not implemented as per-net in the initial network
namespace support by Mario Kicherer (8e8cda6d

).
This patch enables the CAN gateway to be used in different namespaces.

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