can: canxl: update CAN infrastructure for CAN XL frames

- add new ETH_P_CANXL ethernet protocol type
- update skb checks for CAN XL
- add alloc_canxl_skb() which now needs a data length parameter
- introduce init_can_skb_reserve() to reduce code duplication

Acked-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
Link: https://lore.kernel.org/all/20220912170725.120748-6-socketcan@hartkopp.net
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>