A parent process can change a few pages after forking a child and
all this pages should not be avaliable from the child.

Each vma has a bitmap of existent pages. Parent's and child's bitmaps
can be compared and all pages which are not present in a child bitmap
are dropped.

v2: don't check page_bitmap on NULL
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>