sched/fair: Tweak pick_next_entity()

Currently, pick_next_entity(...) has the following structure
(simplified):

  [...]
  if (last_buddy_ok())
    result = last_buddy;
  if (next_buddy_ok())
    result = next_buddy;
  [...]

The intended behavior is to prefer next buddy over last buddy;
the current code somewhat obfuscates this, and also wastes
cycles checking the last buddy when eventually the next buddy is
picked up.

So this patch refactors two 'ifs' above into

  [...]
  if (next_buddy_ok())
      result = next_buddy;
  else if (last_buddy_ok())
      result = last_buddy;
  [...]

Signed-off-by: Peter Oskolkov <posk@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Vincent Guittot <vincent.guitttot@linaro.org>
Link: https://lkml.kernel.org/r/20200930173532.1069092-1-posk@google.com