CVE-2024-35887 ax25: fix use-after-free bugs caused by ax25_ds_del_timer

In the Linux kernel, the following vulnerability has been resolved:

ax25: fix use-after-free bugs caused by ax25_ds_del_timer

When the ax25 device is detaching, the ax25_dev_device_down()
calls ax25_ds_del_timer() to cleanup the slave_timer. When
the timer handler is running, the ax25_ds_del_timer() that
calls del_timer() in it will return directly. As a result,
the use-after-free bugs could happen, one of the scenarios
is shown below:

  (Thread 1)          |      (Thread 2)
                      | ax25_ds_timeout()

ax25_dev_device_down() |
ax25_ds_del_timer() |
del_timer() |
ax25_dev_put() //FREE |
| ax25_dev-> //USE

In order to mitigate bugs, when the device is detaching, use
timer_shutdown_sync() to stop the timer.