This change affects the way that secret, configmap, downwardAPI and projected
volumes (which all use the same underlying code) implement their data update
functionality.
* Instead of creating a subdirectory hierarchy that itself
will contain symlinks to each actual data file, create only
symlinks to items in the root of the volume, whether they
be files or directories.
* Rather than comparing the user-visible data directory
to see if an update is needed, compare with the current
version of the data directory.
* Fix data dir timestamp format year
* Create ..data symlink even when a data volume has no data so
consumers can have simplified update watch logic.
This implements the proposal in:
docs/proposals/secret-configmap-downwarapi-file-mode.md
Fixes: #28317.
The mounttest image is updated so it returns the permissions of the linked file
and not the symlink itself.
