Broke `warn_on_host_root`

[omertuc]

With #137 warn_on_host_root is broken

Running command

sudo podman run -it --rm --privileged -v /dev:/dev -v /var/lib/containers:/var/lib/containers -v /:/target --pid=host --security-opt label=type:unconfined_t -e RUST_LOG=trace quay.io/otuchfel/bootc:latest bash -c 'diff <(stat -f -c %i /target/sysroot) <(stat -f -c %i /proc/1/root/) --side-by-side`

On a bootc booted system

fc0400000000						      |	70000000000

On a normal system

(slightly modified to look at /target rather than /target/sysroot which is what bootc will do)

4a902cc3058b9cd6						4a902cc3058b9cd6

On FCOS

fc0400000000						      |	0

[omertuc]

The solution would be to use the original user provided root_path for the fd passed to warn_on_host_root, rather than the modified one, as that will always match /proc/0/root's fsid (in ostree systems systemd is running with the deployment root as its root, and this is what we have mounted as /:/target)

[omertuc]

I will wait for #905 to merge before pushing a fix, as they're surely going to conflict.

[cgwalters]

I think this is fixed?