ALSA: mtpav: shut down output timer before card teardown

In the Linux kernel, the following vulnerability has been resolved: ALSA: mtpav: shut down output timer before card teardown snd_mtpav_output_timer() rearms chip->timer while holding chip->spinlock and accesses the card-private mtpav state. snd_mtpav_free() currently takes the same lock and calls timer_delete() when the timer is active. This only removes a pending timer; it does not wait for a callback that is already running and does not prevent the callback from rearming the timer. A callback running on another CPU can therefore continue after snd_mtpav_free() releases the lock and access the card-private state while the card is being torn down. It can also rearm the timer after timer_delete() has returned. Call timer_shutdown_sync() without holding chip->spinlock. This waits for any running callback to finish and prevents further rearming before the card-private mtpav state is released.