um: time-travel: Return the sequence number in ACK messages

For external time travel, the protocol says to return the
incoming sequence number in the ACK message to aid debugging,
so do that.
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Acked-By: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Signed-off-by: Richard Weinberger <richard@nod.at>

arch/um/kernel/time.c

@@ -106,6 +106,7 @@ static void time_travel_handle_message(struct um_timetravel_msg *msg,
 		break;
 	}
 
+	resp.seq = msg->seq;
 	os_write_file(time_travel_ext_fd, &resp, sizeof(resp));
 }