can: netlink: add userland error messages

Use NL_SET_ERR_MSG() and NL_SET_ERR_MSG_FMT() to return meaningful
error messages to the userland whenever a -EOPNOTSUPP error is
returned due to a failed validation of the CAN netlink arguments.

Signed-off-by: Vincent Mailhol <mailhol@kernel.org>
Link: https://patch.msgid.link/20250923-canxl-netlink-prep-v4-20-e720d28f66fe@kernel.org


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