!685 unlock driver when clean up driver failed

Merge pull request !685 from Zhangxiaoyu/master

src/daemon/modules/image/oci/storage/layer_store/graphdriver/driver.c

@@ -509,6 +509,7 @@ int graphdriver_cleanup(void)
 
     if (g_graphdriver->ops->clean_up(g_graphdriver) != 0) {
         ret = -1;
+        driver_unlock();
         goto out;
     }