sound: rawmidi: fix checking of O_APPEND when opening MIDI device

Commit 9a1b64ca in 2.6.30 dropped the
check that a substream must already have been opened with O_APPEND to be
able to open it a second time.

This would make it possible for a substream to be switched to append
mode, which would mean that non-atomic writes would fail unexpectedly.
Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Cc: <stable@kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/core/rawmidi.c

@@ -248,7 +248,8 @@ static int assign_substream(struct snd_rawmidi *rmidi, int subdevice,
 	list_for_each_entry(substream, &s->substreams, list) {
 		if (substream->opened) {
 			if (stream == SNDRV_RAWMIDI_STREAM_INPUT ||
-			    !(mode & SNDRV_RAWMIDI_LFLG_APPEND))
+			    !(mode & SNDRV_RAWMIDI_LFLG_APPEND) ||
+			    !substream->append)
 				continue;
 		}
 		if (subdevice < 0 || subdevice == substream->number) {