ALSA: Use del_timer_sync() before freeing timer

The current code for freeing the emux timer is extremely dangerous:

  CPU0				CPU1
  ----				----
snd_emux_timer_callback()
			    snd_emux_free()
			      spin_lock(&emu->voice_lock)
			      del_timer(&emu->tlist); <-- returns immediately
			      spin_unlock(&emu->voice_lock);
			      [..]
			      kfree(emu);

  spin_lock(&emu->voice_lock);

 [BOOM!]

Instead just use del_timer_sync() which will wait for the timer to finish
before continuing. No need to check if the timer is active or not when
doing so.

This doesn't fix the race of a possible re-arming of the timer, but at
least it won't use the data that has just been freed.

[ Fixed unused variable warning by tiwai ]

Cc: stable@vger.kernel.org
Fixes: 1da177e4 ("Linux-2.6.12-rc2")
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20221026231236.6834b551@gandalf.local.homeSigned-off-by: Takashi Iwai <tiwai@suse.de>

sound/synth/emux/emux.c

@@ -126,15 +126,10 @@ EXPORT_SYMBOL(snd_emux_register);
  */
 int snd_emux_free(struct snd_emux *emu)
 {
-	unsigned long flags;
-
 	if (! emu)
 		return -EINVAL;
 
-	spin_lock_irqsave(&emu->voice_lock, flags);
-	if (emu->timer_active)
-		del_timer(&emu->tlist);
-	spin_unlock_irqrestore(&emu->voice_lock, flags);
+	del_timer_sync(&emu->tlist);
 
 	snd_emux_proc_free(emu);
 	snd_emux_delete_virmidi(emu);