cgroups: fix cgroup subsystem mounting

Originally, we aborted all of the cgroup setup if /sys/fs/cgroup/openrc
was already mounted. This  caused an issue in lxc containers, so we
should always allow the subsystems to be mounted.

X-Gentoo-Bug: 520606
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=520606