tcp: Don't discard window information on keep-alive segments

It looks like a detail, but it's critical if we're dealing with
somebody, such as near-future self, using TCP_REPAIR to migrate TCP
connections in the guest or container.

The last packet sent from the 'source' process/guest/container
typically reports a small window, or zero, because the guest/container
hadn't been draining it for a while.

The next packet, appearing as the target sets TCP_REPAIR_OFF on the
migrated socket, is a keep-alive (also called "window probe" in CRIU
or TCP_REPAIR-related code), and it comes with an updated window
value, reflecting the pre-migration "regular" value.

If we ignore it, it might take a while/forever before we realise we
can actually restart sending.

Fixes: 238c69f9af ("tcp: Acknowledge keep-alive segments, ignore them for the rest")
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

tcp.c

@@ -1664,8 +1664,10 @@ static int tcp_data_from_tap(const struct ctx *c, struct tcp_tap_conn *conn,
 			tcp_send_flag(c, conn, ACK);
 			tcp_timer_ctl(c, conn);
 
-			if (p->count == 1)
+			if (p->count == 1) {
+				tcp_tap_window_update(conn, ntohs(th->window));
 				return 1;
+			}
 
 			continue;
 		}