sched/fair: Adhere to place_entity() constraints

Mike reports that commit 6d71a9c6 ("sched/fair: Fix EEVDF entity
placement bug causing scheduling lag") relies on commit 4423af84
("sched/fair: optimize the PLACE_LAG when se->vlag is zero") to not
trip a WARN in place_entity().

What happens is that the lag of the very last entity is 0 per
definition -- the average of one element matches the value of that
element. Therefore place_entity() will match the condition skipping
the lag adjustment:

  if (sched_feat(PLACE_LAG) && cfs_rq->nr_queued && se->vlag) {

Without the 'se->vlag' condition -- it will attempt to adjust the zero
lag even though we're inserting into an empty tree.

Notably, we should have failed the 'cfs_rq->nr_queued' condition, but
don't because they didn't get updated.

Additionally, move update_load_add() after placement() as is
consistent with other place_entity() users -- this change is
non-functional, place_entity() does not use cfs_rq->load.

Fixes: 6d71a9c6 ("sched/fair: Fix EEVDF entity placement bug causing scheduling lag")
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reported-by: Mike Galbraith <efault@gmx.de>
Signed-off-by: "Peter Zijlstra (Intel)" <peterz@infradead.org>
Signed-off-by: Mike Galbraith <efault@gmx.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/c216eb4ef0e0e0029c600aefc69d56681cee5581.camel@gmx.de