I got this patch from Zou Nan hai to resolve the problem that
Andrew Morton has been having booting his ia64 system.  The code
is quite a bit cleaner as he has reduced the number of different
definitions of pfn_valid/page_to_pfn/pfn_to_page, and left the
only remaining ones next to each other in page.h ... so this
should be easier to maintain going forward.
Signed-off-by: Tony Luck <tony.luck@intel.com>