Snapshot.Root, though is a subclass of INodeDirectory, is only used to indicate the root of a snapshot. In the meanwhile, AbstractINodeDiff#snapshotINode is used as copies recording the original state of an INode. Thus we should not put them into INodeMap.
Currently when loading FSImage we did not check the type of inode and wrongly put these two types of nodes into INodeMap. This may replace the nodes that should stay in INodeMap.