x86/mm: Ensure liveness of pages involved in a guest page table walk

x86/mm: Ensure liveness of pages involved in a guest page table walk

Instead of risking deadlock by holding onto the gfn's acquired during
a guest page table walk, acquire an extra reference within the get_gfn/
put_gfn critical section, and drop the extra reference when done with
the map. This ensures liveness of the map, i.e. the underlying page
won't be paged out.

Signed-off-by: Andres Lagar-Cavilla <andres@lagarcavilla.org>
Acked-by: Tim Deegan <tim@xen.org>
Committed-by: Tim Deegan <tim@xen.org>