imagemounter: Avoid error when whiteout target doesn't exist in the parent layers

[ktock]

If we find a whiteout and the target (already) doesn't exist in the parent layers, we don't return error but just log it and continue processing.

This fixes the recent unpacking errors like the following:

parent node of whiteout "var/cache/apt/archives/.wh..wh..opq" is not found: Node "archives" not found