https://bugzilla.redhat.com/show_bug.cgi?id=1591732

On kernels without device mapper support there won't be
/dev/mapper/control. Therefore it doesn't make much sense to
put it into devices CGroup.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>