backing-dev: use kstrto* in preference to simple_strtoul

Fix checkpatch warnings:

WARNING: consider using kstrto* in preference to simple_strtoul

for the below sys entry parsers:
/sys/block/<block device>/bdi/read_ahead_kb
/sys/block/<block device>/bdi/max_ratio
/sys/block/<block device>/bdi/min_ratio
Signed-off-by: Namjae Jeon <linkinjeon@gmail.com>
Signed-off-by: Vivek Trivedi <vtrivedi018@gmail.com>

mm/backing-dev.c

@@ -158,16 +158,16 @@ static ssize_t read_ahead_kb_store(struct device *dev,
 				  const char *buf, size_t count)
 {
 	struct backing_dev_info *bdi = dev_get_drvdata(dev);
-	char *end;
 	unsigned long read_ahead_kb;
-	ssize_t ret = -EINVAL;
+	ssize_t ret;
 
-	read_ahead_kb = simple_strtoul(buf, &end, 10);
-	if (*buf && (end[0] == '\0' || (end[0] == '\n' && end[1] == '\0'))) {
-		bdi->ra_pages = read_ahead_kb >> (PAGE_SHIFT - 10);
-		ret = count;
-	}
-	return ret;
+	ret = kstrtoul(buf, 10, &read_ahead_kb);
+	if (ret < 0)
+		return ret;
+
+	bdi->ra_pages = read_ahead_kb >> (PAGE_SHIFT - 10);
+
+	return count;
 }
 
 #define K(pages) ((pages) << (PAGE_SHIFT - 10))
@@ -187,16 +187,17 @@ static ssize_t min_ratio_store(struct device *dev,
 		struct device_attribute *attr, const char *buf, size_t count)
 {
 	struct backing_dev_info *bdi = dev_get_drvdata(dev);
-	char *end;
 	unsigned int ratio;
-	ssize_t ret = -EINVAL;
+	ssize_t ret;
+
+	ret = kstrtouint(buf, 10, &ratio);
+	if (ret < 0)
+		return ret;
+
+	ret = bdi_set_min_ratio(bdi, ratio);
+	if (!ret)
+		ret = count;
 
-	ratio = simple_strtoul(buf, &end, 10);
-	if (*buf && (end[0] == '\0' || (end[0] == '\n' && end[1] == '\0'))) {
-		ret = bdi_set_min_ratio(bdi, ratio);
-		if (!ret)
-			ret = count;
-	}
 	return ret;
 }
 BDI_SHOW(min_ratio, bdi->min_ratio)
@@ -205,16 +206,17 @@ static ssize_t max_ratio_store(struct device *dev,
 		struct device_attribute *attr, const char *buf, size_t count)
 {
 	struct backing_dev_info *bdi = dev_get_drvdata(dev);
-	char *end;
 	unsigned int ratio;
-	ssize_t ret = -EINVAL;
+	ssize_t ret;
+
+	ret = kstrtouint(buf, 10, &ratio);
+	if (ret < 0)
+		return ret;
+
+	ret = bdi_set_max_ratio(bdi, ratio);
+	if (!ret)
+		ret = count;
 
-	ratio = simple_strtoul(buf, &end, 10);
-	if (*buf && (end[0] == '\0' || (end[0] == '\n' && end[1] == '\0'))) {
-		ret = bdi_set_max_ratio(bdi, ratio);
-		if (!ret)
-			ret = count;
-	}
 	return ret;
 }
 BDI_SHOW(max_ratio, bdi->max_ratio)