migration_call(CPU_DEAD): use spin_lock_irq() instead of task_rq_lock()

Change migration_call(CPU_DEAD) to use direct spin_lock_irq() instead of
task_rq_lock(rq->idle), rq->idle can't change its task_rq().

This makes the code a bit more symmetrical with migrate_dead_tasks()'s path
which uses spin_lock_irq/spin_unlock_irq.
Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Cliff Wickman <cpw@sgi.com>
Cc: Gautham R Shenoy <ego@in.ibm.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Srivatsa Vaddagiri <vatsa@in.ibm.com>
Cc: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/sched.c

@@ -5457,14 +5457,14 @@ migration_call(struct notifier_block *nfb, unsigned long action, void *hcpu)
 		kthread_stop(rq->migration_thread);
 		rq->migration_thread = NULL;
 		/* Idle task back to normal (off runqueue, low prio) */
-		rq = task_rq_lock(rq->idle, &flags);
+		spin_lock_irq(&rq->lock);
 		update_rq_clock(rq);
 		deactivate_task(rq, rq->idle, 0);
 		rq->idle->static_prio = MAX_PRIO;
 		__setscheduler(rq, rq->idle, SCHED_NORMAL, 0);
 		rq->idle->sched_class = &idle_sched_class;
 		migrate_dead_tasks(cpu);
-		task_rq_unlock(rq, &flags);
+		spin_unlock_irq(&rq->lock);
 		migrate_nr_uninterruptible(rq);
 		BUG_ON(rq->nr_running != 0);