From: Jake Moilanen <moilanen@austin.ibm.com>

In xics_get_irq(), for a real-to-virt irq lookup, go down the
slowpath by looking through the entire virt_irq_to_real_map array
if take a miss on the radix tree.  This is possible, when an
interrupt is taken before the driver has called request_irq() (eg IDE).