Don't show / as full or report underlying partition free space (was: df tool shows / is a 32MB partition)

Where this should be fixed

This will likely need changes in EROFS/overlayfs/composefs.

See: https://github.com/composefs/composefs/issues/420

---

Original issue text

I assume this is due to the way the Fedora atomic file systems are put together, but it would break a lot of user's muscle memory when dealing with /.

❯ df -h
Filesystem      Size  Used Avail Use% Mounted on
composefs        32M   32M     0 100% /

It also interferes with file managers like Dolphin that show / as full: