can: enable CAN XL for virtual CAN devices by default

In commit 97edec3a ("can: enable CAN FD for virtual CAN devices by
default"), vcan and vxcan default MTU was set to CANFD_MTU by default.
The reason was that users were confused on how to activate CAN FD on
virtual interfaces.

Following the introduction of CAN XL, the same logic should be
applied. Set the MTU to CANXL_MTU by default.

The users who really wish to use a Classical CAN only or a CAN FD
virtual device can do respectively:

  $ ip link set vcan0 mtu 16

or

  $ ip link set vcan0 mtu 72

to force the old behaviour.

Signed-off-by: Vincent Mailhol <mailhol@kernel.org>
Link: https://patch.msgid.link/20250923-can-fix-mtu-v3-4-581bde113f52@kernel.org


Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>