behavior around parent directory needs clarification

[stevvooe]

Layers may need to define parent directories or the directories will get default permissions. We may need to bolster https://github.com/opencontainers/image-spec/blob/master/layer.md to reflect this detail. Effectively, it should require that if a file has been modified, the tar should include chain of parent directories.

Please see details in containerd/containerd#1723.