agent: reduce reference count for failed mount

The kata agent adds a reference for each storage object before mount
and skip mount again if the storage object is known. We need to
remove the object reference if mount fails.

Fixes: #5364

Signed-off-by: Feng Wang <feng.wang@databricks.com>

src/agent/src/mount.rs

@@ -779,16 +779,20 @@ pub async fn add_storages(
             }
         };
 
-        // Todo need to rollback the mounted storage if err met.
-
-        if res.is_err() {
-            error!(
-                logger,
-                "add_storages failed, storage: {:?}, error: {:?} ", storage, res
-            );
-        }
-
-        let mount_point = res?;
+        let mount_point = match res {
+            Err(e) => {
+                error!(
+                    logger,
+                    "add_storages failed, storage: {:?}, error: {:?} ", storage, e
+                );
+                let mut sb = sandbox.lock().await;
+                sb.unset_sandbox_storage(&storage.mount_point)
+                    .map_err(|e| warn!(logger, "fail to unset sandbox storage {:?}", e))
+                    .ok();
+                return Err(e);
+            }
+            Ok(m) => m,
+        };
 
         if !mount_point.is_empty() {
             mount_list.push(mount_point);