CONFIG_DEBUG_PAGEALLOC generates WARN_ON() in ppc32 hash

[Kumar Gala]

Ben,

If we have CONFIG_DEBUG_PAGEALLOC enabled we end up hitting the  
WARN_ON() in set_pte_at() when we boot.  This appears because we've  
already setup pte's via mapin_ram() after which we end up calling  
kernel_map_pages().

So I'm not sure if __change_page_attr() should be doing something  
different or if the WARN_ON() should be conditional on ! 
CONFIG_DEBUG_PAGEALLOC

- k