page-server: don't return a server pid from page-server

This patch fixes an exit code if a page-server is started
as a daemon. Now the criu exits with a pid of a page server,
it's meaningless, because pid is usually bigger than 128.

travis-ci: success for series starting with [1/2] page-server: don't return a server pid from page-server
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

criu/page-xfer.c

@@ -731,7 +731,7 @@ int cr_page_server(bool daemon_mode, int cfd)
 no_server:
 	ret = run_tcp_server(daemon_mode, &ask, cfd, sk);
 	if (ret != 0)
-		return ret;
+		return ret > 0 ? 0 : -1;
 
 	if (ask >= 0)
 		ret = page_server_serve(ask);