sched/core: uclamp: Add system default clamps

Tasks without a user-defined clamp value are considered not clamped
and by default their utilization can have any value in the
[0..SCHED_CAPACITY_SCALE] range.

Tasks with a user-defined clamp value are allowed to request any value
in that range, and we unconditionally enforce the required clamps.
However, a "System Management Software" could be interested in limiting
the range of clamp values allowed for all tasks.

Add a privileged interface to define a system default configuration via:

  /proc/sys/kernel/sched_uclamp_util_{min,max}

which works as an unconditional clamp range restriction for all tasks.

The default configuration allows the full range of SCHED_CAPACITY_SCALE
values for each clamp index. If otherwise configured, a task specific
clamp is always capped by the corresponding system default value.

Do that by tracking, for each task, the "effective" clamp value and
bucket the task has been actual refcounted in at enqueue time. This
allows to lazy aggregate "requested" and "system default" values at
enqueue time and simplify refcounting updates at dequeue time.

The cached bucket ids are used to avoid (relatively) more expensive
integer divisions every time a task is enqueued.

An active flag is used to report when the "effective" value is valid and
thus the task actually refcounted in the corresponding rq's bucket.

Signed-off-by: Patrick Bellasi <patrick.bellasi@arm.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>

---
Changes in v7:
 Message-ID: <20190124123009.2yulcf25ld66popd@e110439-lin>
 - make system defaults to support a "nice" policy where a task, for
   each clamp index, can get only "up to" what allowed by the system
   default setting, i.e. tasks are always allowed to request for less