ALSA: hda: fix a possible null-pointer dereference due to data race in snd_hdac_regmap_sync()

The variable codec->regmap is often protected by the lock
codec->regmap_lock when is accessed. However, it is accessed without
holding the lock when is accessed in snd_hdac_regmap_sync():

  if (codec->regmap)

In my opinion, this may be a harmful race, because if codec->regmap is
set to NULL right after the condition is checked, a null-pointer
dereference can occur in the called function regcache_sync():

  map->lock(map->lock_arg); --> Line 360 in drivers/base/regmap/regcache.c

To fix this possible null-pointer dereference caused by data race, the
mutex_lock coverage is extended to protect the if statement as well as the
function call to regcache_sync().

[ Note: the lack of the regmap_lock itself is harmless for the current
  codec driver implementations, as snd_hdac_regmap_sync() is only for
  PM runtime resume that is prohibited during the codec probe.
  But the change makes the whole code more consistent, so it's merged
  as is -- tiwai ]

Reported-by: BassCheck <bass@buaa.edu.cn>
Signed-off-by: Tuo Li <islituo@gmail.com>
Link: https://lore.kernel.org/r/20230703031016.1184711-1-islituo@gmail.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/hda/hdac_regmap.c

@@ -596,10 +596,9 @@ EXPORT_SYMBOL_GPL(snd_hdac_regmap_update_raw_once);
  */
 void snd_hdac_regmap_sync(struct hdac_device *codec)
 {
-	if (codec->regmap) {
-		mutex_lock(&codec->regmap_lock);
+	mutex_lock(&codec->regmap_lock);
+	if (codec->regmap)
 		regcache_sync(codec->regmap);
-		mutex_unlock(&codec->regmap_lock);
-	}
+	mutex_unlock(&codec->regmap_lock);
 }
 EXPORT_SYMBOL_GPL(snd_hdac_regmap_sync);