memory hotplug: fix page_zone() calculation in test_pages_isolated()
My last bugfix here (adding zone->lock) introduced a new problem: Using
page_zone(pfn_to_page(pfn)) to get the zone after the for() loop is wrong.
pfn will then be >= end_pfn, which may be in a different zone or not
present at all. This may lead to an addressing exception in page_zone()
or spin_lock_irqsave().
Now I use __first_valid_page() again after the loop to find a valid page
for page_zone().