kdb: use srcu console list iterator

Guarantee safe iteration of the console list by using SRCU.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Reviewed-by: Aaron Tomlin <atomlin@atomlin.com>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20221116162152.193147-15-john.ogness@linutronix.de