sh: push-switch: Reorder cleanup operations to avoid use-after-free bug

The original code puts flush_work() before timer_shutdown_sync()
in switch_drv_remove(). Although we use flush_work() to stop
the worker, it could be rescheduled in switch_timer(). As a result,
a use-after-free bug can occur. The details are shown below:

      (cpu 0)                    |      (cpu 1)
switch_drv_remove()              |
 flush_work()                    |
  ...                            |  switch_timer // timer
                                 |   schedule_work(&psw->work)
 timer_shutdown_sync()           |
 ...                             |  switch_work_handler // worker
 kfree(psw) // free              |
                                 |   psw->state = 0 // use

This patch puts timer_shutdown_sync() before flush_work() to
mitigate the bugs. As a result, the worker and timer will be
stopped safely before the deallocate operations.

Fixes: 9f5e8eee ("sh: generic push-switch framework.")
Signed-off-by: Duoming Zhou <duoming@zju.edu.cn>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
Link: https://lore.kernel.org/r/20230802033737.9738-1-duoming@zju.edu.cn


Signed-off-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>