This is a minimal "get it working again" patch; there are plans a
foot to re-jig the page table code to work better with Ingo Molnar's
changes.  These same plans also allow the ARM page tabkes to fit
into Rik van Riel's rmap significantly better.

(We're currently abusing the struct page * returned from pte_alloc_one,
treating it as if it were the same as a pte_t *)