HMP: Avoid using the cpu stopper to stop runnable tasks

HMP: Avoid using the cpu stopper to stop runnable tasks

When migrating a runnable task, we use the CPU stopper on
the source CPU to ensure that the task to be moved is not
currently running. Before this patch, all forced migrations
(up, offload, idle pull) use the stopper for every migration.

Using the CPU stopper is mandatory only when a task is currently
running on a CPU.  Otherwise tasks can be moved by locking the
source and destination run queues.

This patch checks to see if the task to be moved are currently
running.  If not the task is moved directly without using the
stopper thread.

Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: Jon Medhurst <tixy@linaro.org>