xen,pod: Only sweep in an emergency, and only for 4k pages

xen,pod: Only sweep in an emergency, and only for 4k pages

Testing has shown that doing sweeps for superpages slows down boot
significantly, but does not result in a significantly higher number of
superpages after boot.  Early sweeping for 4k pages causes superpages
to be broken up unnecessarily.

Only sweep if we're really out of memory.

v2:
 - Move unrelated code-motion hunk to another patch
v3:
 - Remove now-unused reclaim_super from pod struct

Signed-off-by: George Dunlap <george.dunlap@eu.citrix.com>
Acked-by: Tim Deegan <tim@xen.org>
Committed-by: Tim Deegan <tim@xen.org>