sched/core: Do not free task while holding rq lock

Clearing the hmp request can cause a task to be freed. When a task is
freed the free call might wake up a kworker which will cause a
spinlock lockup (rq lock). Fix this by avoiding calling put_task_struct
when holding the rq lock.

In addition move call to clear_hmp_request out of stopper thread context
since it is not necessary to do this on the cpu being isolated.

Change-Id: Ie577db4701a88849560df385869ff7cf73695a05
Signed-off-by: Olav Haugan <ohaugan@codeaurora.org>