gpio: cdev: put emitting the line state events on a workqueue

In order to allow line state notifications to be emitted from atomic
context (for instance: from gpiod_direction_input/output()), we must
stop calling any sleeping functions in lineinfo_changed_notify(). To
that end let's use the new workqueue.

Let's atomically allocate small structures containing the required data
and fill it with information immediately upon being notified about the
change except for the pinctrl state which will be retrieved later from
process context. We can pretty reliably do this as pin functions are
typically set once per boot.

Let's make sure to bump the reference count of GPIO device and the GPIO
character device file descriptor to keep both alive until the event was
queued.

Reviewed-by: Kent Gibson <warthog618@gmail.com>
Link: https://lore.kernel.org/r/20241018-gpio-notify-in-kernel-events-v5-6-c79135e58a1c@linaro.org


Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>