sched: Fold sched_class::switch{ing,ed}_{to,from}() into the change pattern

sched: Fold sched_class::switch{ing,ed}_{to,from}() into the change pattern

Add {DE,EN}QUEUE_CLASS and fold the sched_class::switch* methods into
the change pattern. This completes and makes the pattern more
symmetric.

This changes the order of callbacks slightly:

  OLD                              NEW
				|
				|  switching_from()
  dequeue_task();		|  dequeue_task()
  put_prev_task();		|  put_prev_task()
				|  switched_from()
				|
  ... change task ...		|  ... change task ...
				|
  switching_to();		|  switching_to()
  enqueue_task();		|  enqueue_task()
  set_next_task();		|  set_next_task()
  prev_class->switched_from()	|
  switched_to()			|  switched_to()
				|

Notably, it moves the switched_from() callback right after the
dequeue/put. Existing implementations don't appear to be affected by
this change in location -- specifically the task isn't enqueued on the
class in question in either location.

Make (CLASS)^(SAVE|MOVE), because there is nothing to save-restore
when changing scheduling classes.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Juri Lelli <juri.lelli@redhat.com>
Acked-by: Tejun Heo <tj@kernel.org>
Acked-by: Vincent Guittot <vincent.guittot@linaro.org>