vdso: move VMAs traverse cycle in vdso_proxify()

Reduce __export_restore_task(), which is fat and too long,
drop needless parameter from vdso_proxify() and drop also
that find-tuple logic, which isn't needed really.

Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>