sched: Guarantee task priority in pick_next_task()

Michael spotted that the idle_balance() push down created a task
priority problem.

Previously, when we called idle_balance() before pick_next_task() it
wasn't a problem when -- because of the rq->lock droppage -- an rt/dl
task slipped in.

Similarly for pre_schedule(), rt pre-schedule could have a dl task
slip in.

But by pulling it into the pick_next_task() loop, we'll not try a
higher task priority again.

Cure this by creating a re-start condition in pick_next_task(); and
triggering this from pick_next_task_{rt,fair}().

It also fixes a live-lock where we get stuck in pick_next_task_fair()
due to idle_balance() seeing !0 nr_running but there not actually
being any fair tasks about.

Reported-by: Michael Wang <[email protected]>
Fixes: 38033c37faab ("sched: Push down pre_schedule() and idle_balance()")
Tested-by: Sasha Levin <[email protected]>
Signed-off-by: Peter Zijlstra <[email protected]>
Cc: Juri Lelli <[email protected]>
Cc: Steven Rostedt <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>

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