sched/fair: rework load_balance

The load_balance algorithm contains some heuristics which have become
meaningless since the rework of the scheduler's metrics like the
introduction of PELT.

Furthermore, load is an ill-suited metric for solving certain task
placement imbalance scenarios. For instance, in the presence of idle CPUs,
we should simply try to get at least one task per CPU, whereas the current
load-based algorithm can actually leave idle CPUs alone simply because the
load is somewhat balanced. The current algorithm ends up creating virtual
and meaningless value like the avg_load_per_task or tweaks the state of a
group to make it overloaded whereas it's not, in order to try to migrate
tasks.

load_balance should better qualify the imbalance of the group and clearly
define what has to be moved to fix this imbalance.

The type of sched_group has been extended to better reflect the type of
imbalance. We now have :
	group_has_spare
	group_fully_busy
	group_misfit_task
	group_asym_capacity
	group_imbalanced
	group_overloaded

Based on the type fo sched_group, load_balance now sets what it wants to
move in order to fix the imbalance. It can be some load as before but also
some utilization, a number of task or a type of task:
	migrate_task
	migrate_util
	migrate_load
	migrate_misfit

This new load_balance algorithm fixes several pending wrong tasks
placement:
- the 1 task per CPU case with asymmetrics system
- the case of cfs task preempted by other class
- the case of tasks not evenly spread on groups with spare capacity

Also the load balance decisions have been consolidated in the 3 functions
below after removing the few bypasses and hacks of the current code:
- update_sd_pick_busiest() select the busiest sched_group.
- find_busiest_group() checks if there is an imbalance between local and
  busiest group.
- calculate_imbalance() decides what have to be moved.

Finally, the now unused field total_running of struct sd_lb_stats has been
removed.

Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>