Udev wants us to not start mdmon under its cgroup (it kills mdmon if we
leave it in udev's cgroup), so as part of the mdmon startup process, set
the child process to be part of the systemd cgroup instead. However, if
we don't have cgroups, or there is no systemd cgroup, then just silently
move on.
Signed-off-by: Doug Ledford <dledford@redhat.com>