metag: cpu hotplug: route_irq: preserve irq mask

The route_irq() function needs to preserve the irq mask by using the
_irqsave/irqrestore variants of raw spin lock functions instead of the
_irq variants. This is because it is called from __cpu_disable() (via
migrate_irqs()), which is called with IRQs disabled, so using the _irq
variants re-enables IRQs.

This appears to have been causing occasional hits of the
BUG_ON(!irqs_disabled()) in __irq_work_run() during CPU hotplug soak
testing:
  BUG: failure at kernel/irq_work.c:122/__irq_work_run()!
Signed-off-by: James Hogan <james.hogan@imgtec.com>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>

arch/metag/kernel/irq.c

@@ -279,11 +279,12 @@ static void route_irq(struct irq_data *data, unsigned int irq, unsigned int cpu)
 {
 	struct irq_desc *desc = irq_to_desc(irq);
 	struct irq_chip *chip = irq_data_get_irq_chip(data);
+	unsigned long flags;
 
-	raw_spin_lock_irq(&desc->lock);
+	raw_spin_lock_irqsave(&desc->lock, flags);
 	if (chip->irq_set_affinity)
 		chip->irq_set_affinity(data, cpumask_of(cpu), false);
-	raw_spin_unlock_irq(&desc->lock);
+	raw_spin_unlock_irqrestore(&desc->lock, flags);
 }
 
 /*