freespacenotifier: Do not warn on full read only root

Original commit message(s)

• freespacenotifier: Do not warn on full read only root

Fixes: 6af29669 [freespacenotifier] impr: no notification for ro filesystems

See: https://pagure.io/fedora-kde/SIG/issue/539

Reason for the change

For Fedora 41, we want to enable composefs for Fedora Kinoite. With composefs, the root mount point will be a read only mount of a small "100% full" EROFS filesystem.

It looks like the logic added to account for this case was not correct. I've "unfolded" the code to make it more readable.

Test plan

I'm working on testing it for Fedora Kinoite 41 with composefs enabled (read only, 100% full, root mount point).

Screenshots or screen recordings

N/A

Bugs fixed

None yet. We have https://pagure.io/fedora-kde/SIG/issue/539.