sched/fair: Apply more PELT fixes

One additional 'rule' for using update_cfs_rq_load_avg() is that one
should call update_tg_load_avg() if it returns true.

Add a bunch of comments to hopefully clarify some of the rules:

 o  You need to update cfs_rq _before_ any entity attach/detach,
    this is important, because while for mathmatical consisency this
    isn't strictly needed, it is required for the physical
    interpretation of the model, you attach/detach _now_.

 o  When you modify the cfs_rq avg, you have to then call
    update_tg_load_avg() in order to propagate changes upwards.

 o  (Fair) entities are always attached, switched_{to,from}_fair()
    deal with !fair. This directly follows from the definition of the
    cfs_rq averages, namely that they are a direct sum of all
    (runnable or blocked) entities on that rq.

It is the second rule that this patch enforces, but it adds comments
pertaining to all of them.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>