leds: Fix LED_OFF brightness race

leds: Fix LED_OFF brightness race

While commit fa15d8c69238 ("leds: Fix set_brightness_delayed() race")
successfully forces led_set_brightness() to be called with LED_OFF at
least once when switching from blinking to LED on state so that
hw-blinking can be disabled, another race remains. Indeed in
led_set_brightness(LED_OFF) followed by led_set_brightness(any)
scenario the following CPU scheduling can happen:

    CPU0                                     CPU1
    ----                                     ----
 set_brightness_delayed() {
   test_and_clear_bit(BRIGHTNESS_OFF)
                                         led_set_brightness(LED_OFF) {
                                           set_bit(BRIGHTNESS_OFF)
					   queue_work()
                                         }
                                         led_set_brightness(any) {
                                           set_bit(BRIGHTNESS)
					   queue_work() //already queued
                                         }
   test_and_clear_bit(BRIGHTNESS)
     /* LED set with brightness any */
 }

 /* From previous CPU1 queue_work() */
 set_brightness_delayed() {
   test_and_clear_bit(BRIGHTNESS_OFF)
     /* LED turned off */
   test_and_clear_bit(BRIGHTNESS)
     /* Clear from previous run, LED remains off */

In that case the led_set_brightness(LED_OFF)/led_set_brightness(any)
sequence will be effectively executed in reverse order and LED will
remain off.

With the introduction of commit 32360bf6a5d4 ("leds: Introduce ordered
workqueue for LEDs events instead of system_wq") the race is easier to
trigger as sysfs brightness configuration does not wait for
set_brightness_delayed() work to finish (flush_work() removal).

Use delayed_set_value to optionnally re-configure brightness after a
LED_OFF. That way a LED state could be configured more that once but
final state will always be as expected. Ensure that delayed_set_value
modification is seen before set_bit() using smp_mb__before_atomic().

Fixes: fa15d8c69238 ("leds: Fix set_brightness_delayed() race")
Signed-off-by: Remi Pommarel <repk@triplefau.lt>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Link: https://lore.kernel.org/r/19c81177059dab7b656c42063958011a8e4d1a66.1740050412.git.repk@triplefau.lt
Signed-off-by: Lee Jones <lee@kernel.org>