ext4: check return value of kstrtoull correctly in reserved_clusters_store

kstrtoull returns 0 on success, however, in reserved_clusters_store we
will return -EINVAL if kstrtoull returns 0, it makes us fail to update
reserved_clusters value through sysfs.

Fixes: 76d33bca5581b1dd5c3157fa168db849a784ada4
Cc: stable@vger.kernel.org # 4.4
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Miao Xie <miaoxie@huawei.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/sysfs.c

@@ -100,7 +100,7 @@ static ssize_t reserved_clusters_store(struct ext4_attr *a,
 	int ret;
 
 	ret = kstrtoull(skip_spaces(buf), 0, &val);
-	if (!ret || val >= clusters)
+	if (ret || val >= clusters)
 		return -EINVAL;
 
 	atomic64_set(&sbi->s_resv_clusters, val);