sched/eevdf: Propagate min_slice up the cgroup hierarchy

In the absence of an explicit cgroup slice configureation, make mixed
slice length work with cgroups by propagating the min_slice up the
hierarchy.

This ensures the cgroup entity gets timely service to service its
entities that have this timing constraint set on them.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>