sched: Update task->on_rq when tasks are moving between runqueues

Task->on_rq has three states:
	0 - Task is not on runqueue (rq)
	1 (TASK_ON_RQ_QUEUED) - Task is on rq
	2 (TASK_ON_RQ_MIGRATING) - Task is on rq but in the
	process of being migrated to another rq

When a task is moving between rqs task->on_rq state should be
TASK_ON_RQ_MIGRATING in order for WALT to account rq's cumulative
runnable average correctly.  Without such state marking for all the
classes, WALT's update_history() would try to fixup task's demand
which was never contributed to any of CPUs during migration.

Change-Id: Iced3428f3924fe8ab5d0075698273ead04f12d5b
Signed-off-by: Olav Haugan <ohaugan@codeaurora.org>
[joonwoop: Reinforced changelog to explain why this is needed by WALT.
           Fixed conflicts in deadline.c]
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>