ALSA: seq: Fix snd_seq_expand_var_event() call to user-space

The recent fix to clear the padding bytes at
snd_seq_expand_var_event() broke the read to user-space with
in_kernel=0 parameter.  For user-space address, it has to use
clear_user() instead of memset().

Fixes: f80e6d6 ("ALSA: seq: Clear padded bytes at expanding events")
Reported-and-tested-by: Ash Holland <ash@sorrel.sh>
Closes: https://lore.kernel.org/r/8a555319-9f31-4ea2-878f-adc338bc40d4@sorrel.sh
Link: https://lore.kernel.org/r/20230905052631.18240-1-tiwai@suse.de
Link: https://lore.kernel.org/r/20230905081210.6731-1-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>

Author: tiwai

sound/core/seq/seq_memory.c

@@ -187,8 +187,13 @@ int snd_seq_expand_var_event(const struct snd_seq_event *event, int count, char
 	err = expand_var_event(event, 0, len, buf, in_kernel);
 	if (err < 0)
 		return err;
-	if (len != newlen)
-		memset(buf + len, 0, newlen - len);
+	if (len != newlen) {
+		if (in_kernel)
+			memset(buf + len, 0, newlen - len);
+		else if (clear_user((__force void __user *)buf + len,
+				    newlen - len))
+			return -EFAULT;
+	}
 	return newlen;
 }
 EXPORT_SYMBOL(snd_seq_expand_var_event);