printk/nbcon/panic: Allow printk kthread to sleep when the system is in panic

printk/nbcon/panic: Allow printk kthread to sleep when the system is in panic

The printk kthread might be running when there is a panic in progress.
But it is not able to acquire the console ownership any longer.

Prevent the desperate attempts to acquire the ownership and allow sleeping
in panic. It would make it behave the same as when there is any CPU
in an emergency context.

Reviewed-by: Andrew Murray <amurray@thegoodpenguin.co.uk>
Reviewed-by: John Ogness <john.ogness@linutronix.de>
Link: https://patch.msgid.link/20250926124912.243464-3-pmladek@suse.com
[pmladek@suse.com: Rebased on top of 6.18-rc1 (panic_in_progress() moved to panic.c)]
Signed-off-by: Petr Mladek <pmladek@suse.com>