PID ussualy means processs ID, but prepare_net_ns works with namespaces.

travis-ci: success for Dump and restore nested network namespaces (rev4)
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>
Reviewed-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>