service: use send_criu_pre_dump_resp() in pre_dump_using_req()

Currently we have a bug, service sends resp of type PRE_DUMP
instead of DUMP. So lets introduce send_criu_pre_dump_resp() and
use it.

Signed-off-by: Ruslan Kuprieiev <kupruser@gmail.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>