ALSA: pcm - Fix drain behavior in non-blocking mode

The current PCM core has the following problems regarding PCM draining
in non-blocking mode:

- the current f_flags isn't checked in snd_pcm_drain(), thus changing
  the mode dynamically via snd_pcm_nonblock() after open doesn't work.
- calling drain in non-blocking mode just return -EAGAIN error, but
  doesn't provide any way to sync with draining.

This patch fixes these issues.
- check file->f_flags in snd_pcm_drain() properly
- when O_NONBLOCK is set, PCM core sets the stream(s) to DRAIN state
  but quits ioctl immediately without waiting the whole drain; the
  caller can sync the drain manually via poll()

Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/core/pcm_lib.c

@@ -197,12 +197,16 @@ static int snd_pcm_update_hw_ptr_post(struct snd_pcm_substream *substream,
 		avail = snd_pcm_capture_avail(runtime);
 	if (avail > runtime->avail_max)
 		runtime->avail_max = avail;
-	if (avail >= runtime->stop_threshold) {
-		if (substream->runtime->status->state == SNDRV_PCM_STATE_DRAINING)
+	if (runtime->status->state == SNDRV_PCM_STATE_DRAINING) {
+		if (avail >= runtime->buffer_size) {
 			snd_pcm_drain_done(substream);
-		else
+			return -EPIPE;
+		}
+	} else {
+		if (avail >= runtime->stop_threshold) {
 			xrun(substream);
-		return -EPIPE;
+			return -EPIPE;
+		}
 	}
 	if (avail >= runtime->control->avail_min)
 		wake_up(&runtime->sleep);