[core/sched/cbs] Add lower-bound saturation for round correction

Without this lower-bound it could happen that, for really long error correcrtion
actions, the next round cycles is defined to be 0.
Since it makes no sense to have a round cycle which is smaller than:
   burst_min_ns * nr_running
we set that value as lower bound.

Signed-off-by: Patrick Bellasi <derkling@gmail.com>
[core/sched/cbs] Switch to resched_task to trigger the scheduler

The resched_task method, provided by the core scheduler, is more safe and
generic since it cares about locking as well as SMP platforms.

Signed-off-by: Patrick Bellasi <derkling@gmail.com>
[core/sched/cbs] Optimization for queued HRTimer ticks

Task ticks calls generated by a scheduler HRTimer are notified to the
scheduling policy via the "queued" flag. In that case, we can avoid to
brother with preemption validation and just return with a resched request.

At the same time, when an HRTImer is pending, we can ignore periodic ticks
and return without rescheduling the current task, which is still consuming
its allotted burst.

Signed-off-by: Patrick Bellasi <derkling@gmail.com>