Vladimir said that a page can be unmapped from one process, but can be
mmaped in another one. In this case we can't understand whether both
processes link to the same page or they don't.

So we can try read both pages to take them from swap.

Cc: Vladimir Davydov <vdavydov@parallels.com>
Reported-by: Parallels' QA
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>