If a vdso page was not accessed before dump, it's not present
and now it doesn't save in a dump image.
The vdso area is restored as VMA_ANON, it isn't an actual vdso more,
so when a not-present vdso page is accessed, a zero page would be mapped.

For this reason we should save all vdso pages in a image.

v2: make code more clear
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>