For some reason I booted a NUMA and SLAB_DEBUG i386 kernel on a non
NUMA 512MB machine.  This caused an oops at bootup in change_page_attr.
The reason was that highmem_start_start page ended up zero and 
that triggered the highmem check in change_page_attr when the
slab debug code would unmap a kernel mapping.

Fix is straightforward: if there is no highmem set highmem_start_page
to max_low_pfn+1