printk: nbcon: Use thread callback if in task context for legacy

printk: nbcon: Use thread callback if in task context for legacy

When printing via console_lock, the write_atomic() callback is
used for nbcon consoles. However, if it is known that the
current context is a task context, the write_thread() callback
can be used instead.

Using write_thread() instead of write_atomic() helps to reduce
large disabled preemption regions when the device_lock does not
disable preemption.

This is mainly a preparatory change to allow avoiding
write_atomic() completely during normal operation if boot
consoles are registered.

As a side-effect, it also allows consolidating the printing
code for legacy printing and the kthread printer.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20240904120536.115780-9-john.ogness@linutronix.de
Signed-off-by: Petr Mladek <pmladek@suse.com>