sched: Ensure the migration task doesn't go away during use

Like sched_migrate_task(), set_cpus_allowed_ptr() should hold
onto the migration thread too.
Signed-off-by: NPeter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

kernel/sched.c

@@ -7078,8 +7078,12 @@ int set_cpus_allowed_ptr(struct task_struct *p, const struct cpumask *new_mask)
 
 	if (migrate_task(p, cpumask_any_and(cpu_online_mask, new_mask), &req)) {
 		/* Need help from migration thread: drop lock and wait. */
+		struct task_struct *mt = rq->migration_thread;
+
+		get_task_struct(mt);
 		task_rq_unlock(rq, &flags);
 		wake_up_process(rq->migration_thread);
+		put_task_struct(mt);
 		wait_for_completion(&req.done);
 		tlb_migrate_finish(p->mm);
 		return 0;