printk: nbcon: Allow drivers to mark unsafe regions and check state

For the write_atomic callback, the console driver may have unsafe
regions that need to be appropriately marked. Provide functions
that accept the nbcon_write_context struct to allow for the driver
to enter and exit unsafe regions.

Also provide a function for drivers to check if they are still the
owner of the console.

Co-developed-by: John Ogness <john.ogness@linutronix.de>
Signed-off-by: John Ogness <john.ogness@linutronix.de>
Signed-off-by: Thomas Gleixner (Intel) <tglx@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20230916192007.608398-9-john.ogness@linutronix.de