sched/core: Introduce set_next_task() helper for better code readability

When we pick the next task, we will do the following for the task:

  1) p->se.exec_start = rq_clock_task(rq);
  2) dequeue_pushable(_dl)_task(rq, p);

When we call set_curr_task(), we also need to do the same thing
above. In rt.c, the code at 1) is in the _pick_next_task_rt()
and the code at 2) is in the pick_next_task_rt(). If we put two
operations in one function, maybe better. So, we introduce a new
function set_next_task(), which is responsible for doing the above.

By introducing the function we can get rid of calling the
dequeue_pushable(_dl)_task() directly(We can call set_next_task())
in pick_next_task() and have better code readability and reuse.
In set_curr_task_rt(), we also can call set_next_task().

Do this things such that we end up with:

  static struct task_struct *pick_next_task(struct rq *rq,
  					    struct task_struct *prev,
  					    struct rq_flags *rf)
  {
  	/* do something else ... */

  	put_prev_task(rq, prev);

  	/* pick next task p */

  	set_next_task(rq, p);

  	/* do something else ... */
  }

put_prev_task() can match set_next_task(), which can make the
code more readable.

Signed-off-by: Muchun Song <smuchun@gmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20181026131743.21786-1-smuchun@gmail.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>