xenpaging: clear page content after evict

xenpaging: clear page content after evict

If the guest happens to read from the gfn while xenpaging is in the process of
evicting the page, the guest may read zeros instead of actual data.
Also if eviction fails the page content will be corrupted and xenpaging wont
attempt to restore the page.

Remove page scrubbing from pager and do it after successful eviction.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Tim Deegan <tim@xen.org>
Committed-by: Tim Deegan <tim@xen.org>