sched: balance callbacks

Mike stumbled over a cute bug where the RT/DL balancing ops caused a bug.

The exact scenario is __sched_setscheduler() changing a (runnable) task from FIFO to OTHER. In swiched_from_rt(), where we do pull_rt_task() we temporarity drop rq->lock. This gap allows regular cfs load-balancing to step in and migrate our task.

However, check_class_changed() will happily continue with switched_to_fair() which assumes our task is still on the old rq and makes the kernel go boom.

Instead of trying to patch this up and make things complicated; simply disallow these methods to drop rq->lock and extend the current post_schedule stuff into a balancing callback list, and use that.

This survives Mike's testcase.

Changes since -v1:
- make SMP=n build,
- cured switched_from_dl()'s cancel_dl_timer().

no real tests on the new parts other than booting.