can: netlink: can_changelink(): fix NULL pointer deref of struct can_priv::do_set_mode

can: netlink: can_changelink(): fix NULL pointer deref of struct can_priv::do_set_mode

Andrei Lalaev reported a NULL pointer deref when a CAN device is
restarted from Bus Off and the driver does not implement the struct
can_priv::do_set_mode callback.

There are 2 code path that call struct can_priv::do_set_mode:
- directly by a manual restart from the user space, via
  can_changelink()
- delayed automatic restart after bus off (deactivated by default)

To prevent the NULL pointer deference, refuse a manual restart or
configure the automatic restart delay in can_changelink() and report
the error via extack to user space.

As an additional safety measure let can_restart() return an error if
can_priv::do_set_mode is not set instead of dereferencing it
unchecked.

Reported-by: Andrei Lalaev <andrey.lalaev@gmail.com>
Closes: https://lore.kernel.org/all/20250714175520.307467-1-andrey.lalaev@gmail.com
Fixes: 39549eef3587 ("can: CAN Network device driver and Netlink interface")
Link: https://patch.msgid.link/20250718-fix-nullptr-deref-do_set_mode-v1-1-0b520097bb96@pengutronix.de
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>