service: Fix page-server start return code checking

Commit 10801f36 (page-server: don't return a server pid from
page-server) broke page-server starting via RPC -- the latter
code checks for return value being <=0 as failure.
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>

criu/cr-service.c

@@ -713,7 +713,7 @@ static int start_page_server_req(int sk, CriuOpts *req)
 		pr_debug("Starting page server\n");
 
 		pid = cr_page_server(true, start_pipe[1]);
-		if (pid <= 0)
+		if (pid < 0)
 			goto out_ch;
 
 		info.pid = pid;