ARM: CPU hotplug: ensure correct ordering of unplug

Don't call idle_task_exit() with interrupts disabled, and ensure
that we have a memory barrier after interrupts are disabled but
before signalling that this CPU has shut down.
Signed-off-by: NRussell King <rmk+kernel@arm.linux.org.uk>

arch/arm/kernel/smp.c

@@ -269,9 +269,11 @@ void __ref cpu_die(void)
 {
 	unsigned int cpu = smp_processor_id();
 
-	local_irq_disable();
 	idle_task_exit();
 
+	local_irq_disable();
+	mb();
+
 	/* Tell __cpu_die() that this CPU is now safe to dispose of */
 	complete(&cpu_died);