metag: use clear_tasks_mm_cpumask()

Checking for process->mm is not enough because process' main thread may
exit or detach its mm via use_mm(), but other threads may still have a
valid mm.

To fix this we would need to use find_lock_task_mm(), which would walk
up all threads and returns an appropriate task (with task lock held).

clear_tasks_mm_cpumask() was introduced in v3.5-rc1 to fix this issue,
so let's use it for metag too.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: "Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Anton Vorontsov <anton.vorontsov@linaro.org>

arch/metag/kernel/smp.c

@@ -276,7 +276,6 @@ static DECLARE_COMPLETION(cpu_killed);
 int __cpuexit __cpu_disable(void)
 {
 	unsigned int cpu = smp_processor_id();
-	struct task_struct *p;
 
 	/*
 	 * Take this CPU offline.  Once we clear this, we can't return,
@@ -296,12 +295,7 @@ int __cpuexit __cpu_disable(void)
 	flush_cache_all();
 	local_flush_tlb_all();
 
-	read_lock(&tasklist_lock);
-	for_each_process(p) {
-		if (p->mm)
-			cpumask_clear_cpu(cpu, mm_cpumask(p->mm));
-	}
-	read_unlock(&tasklist_lock);
+	clear_tasks_mm_cpumask(cpu);
 
 	return 0;
 }