fstab cleanup of `/` does not work

It looks like we regressed after commit d46b072cd218ab114a01d604592b5c42cc1b8866 since the fstab cleanup does not work (anymore).

The `systemd-remount-fs.service` fails with

>mount: /: fsconfig system call failed: overlay: No changes allowed in reconfigure.
>dmesg(1) may have more information after failed mount system call.
>/usr/bin/mount for / exited with exit status 32.

`/etc/fstab` includes
> UUID=d457c96a-3adb-48aa-9bb3-3c34ba1c338b / btrfs subvol=root,compress=zstd:1,ro 0 0

Start by comparing the behavior after commit d46b072cd218ab114a01d604592b5c42cc1b8866 and inspect the handling of the `/etc/fstab` root entry alongside systemd-remount-fs.service. Reproduce the failure with the provided btrfs configuration and verify that cleanup no longer causes the overlay reconfigure error.

Scritto dal modello di indicizzazione a partire dal testo della issue.