sched: core: Skip migrating tasks that aren't enqueued on dead_rq

During migrate_tasks, we have to drop the dead_rq lock in
order to preserve locking order when acquiring task->pi_lock.
This may allow the task to migrate off of dead_rq. Therefore,
don't attempt to migrate such a task again from dead_rq.

Change-Id: Id31b58e231d3dcd7d32e0dc7f264595d60a7c408
Signed-off-by: Vikram Mulukutla <markivx@codeaurora.org>