Otherwise ia64 and sparc64 explode with the new ptwalk
iterators.  The pgd level stuff does not handle virtual
address space holes (sparc64) and region based PGD indexing
(ia64) properly.  It only matters in functions like
clear_page_range() which potentially walk over more than
a single VMA worth of address space.
Signed-off-by: David S. Miller <davem@davemloft.net>