linux: attempt to make rootfs private too

[giuseppe]

commit 6682432 introduced the regression. After that change, crun does not attempt anymore to make the rootfs directory private but starts from its parent directory, causing pivot_root to fail when the rootfs itself is a mountpoint.

Closes: #1514

[packit-as-a-service]

We were not able to find or create Copr project packit/containers-crun-1520 specified in the config with the following error:

Packit received HTTP 500 Internal Server Error from Copr Service. Check the Copr status page: https://copr.fedorainfracloud.org/status/stats/, or ask for help in Fedora Build System matrix channel https://matrix.to/#/#buildsys:fedoraproject.org.

---

Unless the HTTP status code above is >= 500, please check your configuration for:

1. typos in owner and project name (groups need to be prefixed with @)
2. whether the project name doesn't contain not allowed characters (only letters, digits, underscores, dashes and dots must be used)
3. whether the project itself exists (Packit creates projects only in its own namespace)
4. whether Packit is allowed to build in your Copr project
5. whether your Copr project/group is not private

[giuseppe]

please don't merge until we confirm it fixes #1514

[giuseppe]

rebased, I'll merge once CI passes

[flouthoc]

LGTM

[rhatdan]

LGTM