vdso: drop excessive parameter who from vdso_proxify

It's always "dumpee" and is used in two pr_err() messages,
put it right there.

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