can: dev: can_restart(): don't crash kernel if carrier is OK

can: dev: can_restart(): don't crash kernel if carrier is OK

During testing, I triggered a can_restart() with the netif carrier
being OK [1]. The BUG_ON, which checks if the carrier is OK, results
in a fatal kernel crash. This is neither helpful for debugging nor for
a production system.

[1] The root cause is a race condition in can_restart() which will be
fixed in the next patch.

Do not crash the kernel, issue an error message instead, and continue
restarting the CAN device anyway.

Fixes: 39549eef3587 ("can: CAN Network device driver and Netlink interface")
Link: https://lore.kernel.org/all/20231005-can-dev-fix-can-restart-v2-1-91b5c1fd922c@pengutronix.de
Reviewed-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>