[ALSA] pcm core: add prealloc_max file to substream directory to show maximum DMA size

Users ask us many times about the maximum DMA size for PCM devices. This
file gives them a hint in KB.

Signed-off-by: Jaroslav Kysela <perex@suse.cz>
diff --git a/include/sound/pcm.h b/include/sound/pcm.h
index 016c418..ec006ed 100644
--- a/include/sound/pcm.h
+++ b/include/sound/pcm.h
@@ -384,6 +384,7 @@
 	struct snd_info_entry *proc_sw_params_entry;
 	struct snd_info_entry *proc_status_entry;
 	struct snd_info_entry *proc_prealloc_entry;
+	struct snd_info_entry *proc_prealloc_max_entry;
 #endif
 	/* misc flags */
 	unsigned int hw_opened: 1;