Patch from Martin Bligh and Dave Hansen

If a PAE machine has 1G of memory and you set PAGE_OFFSET to 2G, the
kernel will only instantiate a PMD to cover the 2G-3G region.  But
another PMD is needed for the 3G-4G region for the APIC and possibly an
extended vmalloc region.

So the patch changes the code to instantiate PMDs out to the end of
physical memory.

It's a no-op for PAGE_OFFSET=3G, and _could_ be part of the
CONFIG_PAGE_OFFSET patch.  But it seems a reasonable generalisation
anyway.