move_one_page() is calling alloc_one_pte_map() while holding an atomic
kmap for the source pte's page.  But alloc_one_pte_map() can sleep in
the page allocator.

So change move_one_page() to take a peek at the destination pagetables
to work out whether the alloc_one_pte_map() will need to perform page
allocation.  If so, drop the atomic kmap and retake it after allocating
the pte.