ALSA: rawmidi: Drop register_mutex in snd_rawmidi_free()

The register_mutex taken around the dev_unregister callback call in
snd_rawmidi_free() may potentially lead to a mutex deadlock, when OSS
emulation and a hot unplug are involved.

Since the mutex doesn't protect the actual race (as the registration
itself is already protected by another means), let's drop it.

Link: https://lore.kernel.org/r/CAB7eexJP7w1B0mVgDF0dQ+gWor7UdkiwPczmL7pn91xx8xpzOA@mail.gmail.com
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20221011070147.7611-1-tiwai@suse.deSigned-off-by: Takashi Iwai <tiwai@suse.de>

sound/core/rawmidi.c

@@ -1899,10 +1899,8 @@ static int snd_rawmidi_free(struct snd_rawmidi *rmidi)
 
 	snd_info_free_entry(rmidi->proc_entry);
 	rmidi->proc_entry = NULL;
-	mutex_lock(&register_mutex);
 	if (rmidi->ops && rmidi->ops->dev_unregister)
 		rmidi->ops->dev_unregister(rmidi);
-	mutex_unlock(&register_mutex);
 
 	snd_rawmidi_free_substreams(&rmidi->streams[SNDRV_RAWMIDI_STREAM_INPUT]);
 	snd_rawmidi_free_substreams(&rmidi->streams[SNDRV_RAWMIDI_STREAM_OUTPUT]);