page-pipe: move tunable constants into config.h

PIPE_MAX_SIZE is calculated according with the kernel code.
PPB_IOV_BATCH has been taken from my mind.

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>