ASoC: SOF: ipc3-dtrace: Switch to memdup_user_nul() helper

Use memdup_user_nul() helper instead of open-coding to
simplify the code.
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Link: https://lore.kernel.org/r/20230725120247.509422-1-yangyingliang@huawei.comSigned-off-by: Mark Brown <broonie@kernel.org>

sound/soc/sof/ipc3-dtrace.c

@@ -196,15 +196,9 @@ static ssize_t dfsentry_trace_filter_write(struct file *file, const char __user
 		return -EINVAL;
 	}
 
-	string = kmalloc(count + 1, GFP_KERNEL);
-	if (!string)
-		return -ENOMEM;
-
-	if (copy_from_user(string, from, count)) {
-		ret = -EFAULT;
-		goto error;
-	}
-	string[count] = '\0';
+	string = memdup_user_nul(from, count);
+	if (IS_ERR(string))
+		return PTR_ERR(string);
 
 	ret = trace_filter_parse(sdev, string, &num_elems, &elems);
 	if (ret < 0)