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.