Impact: intermittent guest segv/crash fix

I've been seeing random guest bad address crashes and segmentation faults:
bisect led to 4f98a2fe (vmscan: split LRU lists into anon & file sets),
but that's a red herring.

It turns out that lguest never hooked up the pte_update/pte_update_defer
calls, so our ptes were not always in sync.  After the vmscan commit, the
bug became reproducible; now a fsck in a 64MB guest causes reproducible
pagetable corruption.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Cc: jeremy@xensource.com
Cc: virtualization@lists.osdl.org
Cc: stable@kernel.org