parasite-vdso: add casts from uint64_t

As for compatible 32-bit pie size of uint64_t is bigger than size of
unsigned long type, add proper casts, so we could build compatible pie.

Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>