ALSA: seq: virmidi: Fix discarding the unsubscribed output

The recent change to move the virmidi output processing to a work
slightly modified the code to discard the unsubscribed outputs so that
it works without a temporary buffer.  However, this is actually buggy,
and may spew a kernel warning due to the unexpected call of
snd_rawmidi_transmit_ack(), as triggered by syzbot.

This patch takes back to the original code in that part, use a
temporary buffer and simply repeat snd_rawmidi_transmit(), in order to
address the regression.

Fixes: f7debfe5 ("ALSA: seq: virmidi: Offload the output event processing")
Reported-by: syzbot+ec5f605c91812d200367@syzkaller.appspotmail.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/core/seq/seq_virmidi.c

@@ -149,8 +149,9 @@ static void snd_vmidi_output_work(struct work_struct *work)
 	/* discard the outputs in dispatch mode unless subscribed */
 	if (vmidi->seq_mode == SNDRV_VIRMIDI_SEQ_DISPATCH &&
 	    !(vmidi->rdev->flags & SNDRV_VIRMIDI_SUBSCRIBE)) {
-		while (!snd_rawmidi_transmit_empty(substream))
-			snd_rawmidi_transmit_ack(substream, 1);
+		char buf[32];
+		while (snd_rawmidi_transmit(substream, buf, sizeof(buf)) > 0)
+			; /* ignored */
 		return;
 	}