[ALSA] Fix races in PCM OSS emulation

Fixed the race among multiple threads accessing the OSS PCM
instance concurrently by simply introducing a mutex for protecting
a setup of the PCM.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>

include/sound/pcm_oss.h

@@ -56,6 +56,7 @@ struct snd_pcm_oss_runtime {
 	size_t mmap_bytes;
 	char *buffer;				/* vmallocated period */
 	size_t buffer_used;			/* used length from period buffer */
+	struct mutex params_lock;
 #ifdef CONFIG_SND_PCM_OSS_PLUGINS
 	struct snd_pcm_plugin *plugin_first;
 	struct snd_pcm_plugin *plugin_last;