daemon: execdriver: lxc: fix cgroup paths

When running LXC dind (outer docker is started with native driver)
cgroup paths point to `/docker/CID` inside `/proc/self/mountinfo` but
these paths aren't mounted (root is wrong). This fix just discard the
cgroup dir from mountinfo and set it to root `/`.
This patch fixes/skip OOM LXC tests that were failing.
Fix #16520

Signed-off-by: Antonio Murdaca <runcom@linux.com>
Signed-off-by: Antonio Murdaca <amurdaca@redhat.com>