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>