restore: Report error number if PR_SET_MM failed

travis-ci: success for restore: Report error number if PR_SET_MM failed
Reported-by: Andrei Vagin <avagin@virtuozzo.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

criu/pie/restorer.c

@@ -1249,8 +1249,11 @@ long __export_restore_task(struct task_restore_args *args)
 		 * new ones from image file.
 		 */
 		ret |= restore_self_exe_late(args);
-	} else
+	} else {
+		if (ret)
+			pr_err("sys_prctl(PR_SET_MM, PR_SET_MM_MAP) failed with %d\n", (int)ret);
 		sys_close(args->fd_exe_link);
+	}
 
 	if (ret)
 		goto core_restore_end;