It never worked properly before because the pages might simply
be not present in memory at all, so trying to fetch their contents
via splice simply return the number of bytes successfully fetched
which of course different from completed vma size.

Thus correct implementation requires interactions with parasite page
pulling: just read as much as kernel allows us and that's all.

Util it's implemented -- zap it.
Reported-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>