HID: appletb-kbd: run inactivity autodim from workqueues

HID: appletb-kbd: run inactivity autodim from workqueues

The autodim code in hid-appletb-kbd takes backlight_device->ops_lock
via backlight_device_set_brightness() -> mutex_lock() from two
different atomic contexts:

 * appletb_inactivity_timer() is a struct timer_list callback, so it
   runs in softirq context.  Every expiry triggers

     BUG: sleeping function called from invalid context at kernel/locking/mutex.c:591
     Call Trace:
      <IRQ>
      __might_resched
      __mutex_lock
      backlight_device_set_brightness
      appletb_inactivity_timer
      call_timer_fn
      run_timer_softirq

 * reset_inactivity_timer() is called from appletb_kbd_hid_event() and
   appletb_kbd_inp_event().  On real USB hardware these run in
   softirq/IRQ context (URB completion and input-event dispatch).
   When the Touch Bar has already been dimmed or turned off, the
   reset path calls backlight_device_set_brightness() directly to
   restore brightness, producing the same warning.

Both call sites hit the same mutex_lock()-from-atomic bug.  Fix them
together by moving the blocking work onto the system workqueue:

 * Convert the inactivity timer from struct timer_list to
   struct delayed_work; the callback (appletb_inactivity_work) now
   runs in process context where mutex_lock() is legal.
 * Add a dedicated struct work_struct restore_brightness_work and have
   reset_inactivity_timer() schedule it instead of calling
   backlight_device_set_brightness() directly.

Cancel both works synchronously during driver tear-down alongside the
existing backlight reference drop.

The semantics are unchanged (same delays, same state transitions on
dim, turn-off and user activity); only the execution context of the
sleeping call changes.  The timer field and callback are renamed to
match their new type; reset_inactivity_timer() keeps its name because
it is invoked from input event paths that read naturally as "reset
the inactivity timer".

Fixes: 93a0fc489481 ("HID: hid-appletb-kbd: add support for automatic brightness control while using the touchbar")
Cc: stable@vger.kernel.org
Signed-off-by: Sangyun Kim <sangyun.kim@snu.ac.kr>
Signed-off-by: Jiri Kosina <jkosina@suse.com>