powerpc: hard disable irqs in smp_send_stop loop

The hard lockup watchdog can fire under local_irq_disable
on platforms with irq soft masking.
Signed-off-by: NNicholas Piggin <npiggin@gmail.com>
Signed-off-by: NMichael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/smp.c

@@ -574,9 +574,10 @@ static void stop_this_cpu(void *dummy)
 	/* Remove this CPU */
 	set_cpu_online(smp_processor_id(), false);
 
-	local_irq_disable();
+	hard_irq_disable();
+	spin_begin();
 	while (1)
-		;
+		spin_cpu_relax();
 }
 
 void smp_send_stop(void)