From: Benjamin Herrenschmidt <benh@kernel.crashing.org>

The patch below fixes a 2.6 mm problem.  Without this patch, zeromapped
pages are not flushed properly when they are swapped out.

What happens is that the page->index field is zero for page table pages
corresponding to the zeromapped range.  This causes ptep_to_address() to
return an incorrect virtual address with the result that PTEs are never
invalidated at swap-out...

The fix below mirrors the remap_pmd_range() case.