filer store: mongodb deleting children

fix https://github.com/chrislusf/seaweedfs/issues/1303

weed/filer2/mongodb/mongodb_store.go

@@ -159,7 +159,7 @@ func (store *MongodbStore) DeleteEntry(ctx context.Context, fullpath util.FullPa
 func (store *MongodbStore) DeleteFolderChildren(ctx context.Context, fullpath util.FullPath) error {
 
 	where := bson.M{"directory": fullpath}
-	_, err := store.connect.Database(store.database).Collection(store.collectionName).DeleteOne(ctx, where)
+	_, err := store.connect.Database(store.database).Collection(store.collectionName).DeleteMany(ctx, where)
 	if err != nil {
 		return fmt.Errorf("delete %s : %v", fullpath, err)
 	}