[PATCH] small migration thread fix

This fixes a potential problem in the migration thread for the case that
the first CPU in the cpus_allowed mask of a process is offline.

kernel/sched.c

@@ -2108,7 +2108,7 @@ static int migration_thread(void * data)
 		spin_unlock_irqrestore(&rq->lock, flags);
 
 		p = req->task;
-		cpu_dest = __ffs(p->cpus_allowed);
+		cpu_dest = __ffs(p->cpus_allowed & cpu_online_map);
 		rq_dest = cpu_rq(cpu_dest);
 repeat:
 		cpu_src = task_cpu(p);