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: [e-mail] # 4.4
Signed-off-by: Chao Yu <[e-mail]>
Signed-off-by: Miao Xie <[e-mail]>
Signed-off-by: Theodore Ts'o <[e-mail]>

fs/ext4/sysfs.c

···
 	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);