can: gw: add support for CAN FD frames

Introduce CAN FD support which needs an extension of the netlink API to
pass CAN FD type content to the kernel which has a different size to
Classic CAN. Additionally the struct canfd_frame has a new 'flags' element
that can now be modified with can-gw.

The new CGW_FLAGS_CAN_FD option flag defines whether the routing job
handles Classic CAN or CAN FD frames. This setting is very strict at
reception time and enables the new possibilities, e.g. CGW_FDMOD_* and
modifying the flags element of struct canfd_frame, only when
CGW_FLAGS_CAN_FD is set.

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