ALSA: pcm: Add xrun_injection proc entry

This patch adds a new proc entry for PCM substreams to inject an
XRUN.  When a PCM substream is running and any value is written to its
xrun_injection proc file, the driver triggers XRUN.  This is a useful
feature for debugging XRUN and error handling code paths.

Note that this entry is enabled only when CONFIG_SND_PCM_XRUN_DEBUG is
set.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
diff --git a/include/sound/pcm.h b/include/sound/pcm.h
index 29eb09ef..0b8daee 100644
--- a/include/sound/pcm.h
+++ b/include/sound/pcm.h
@@ -416,7 +416,10 @@
 	struct snd_info_entry *proc_status_entry;
 	struct snd_info_entry *proc_prealloc_entry;
 	struct snd_info_entry *proc_prealloc_max_entry;
+#ifdef CONFIG_SND_PCM_XRUN_DEBUG
+	struct snd_info_entry *proc_xrun_injection_entry;
 #endif
+#endif /* CONFIG_SND_VERBOSE_PROCFS */
 	/* misc flags */
 	unsigned int hw_opened: 1;
 };