tcp: Send "empty" handshake ACK before first data segment

Starting from commit 9178a9e346 ("tcp: Always send an ACK segment
once the handshake is completed"), we always send an ACK segment,
without any payload, to complete the three-way handshake while
establishing a connection started from a socket.

We queue that segment after checking if we already have data to send
to the tap, which means that its sequence number is higher than any
segment with data we're sending in the same iteration, if any data is
available on the socket.

However, in tcp_defer_handler(), we first flush "flags" buffers, that
is, we send out segments without any data first, and then segments
with data, which means that our "empty" ACK is sent before the ACK
segment with data (if any), which has a lower sequence number.

This appears to be harmless as the guest or container will generally
reorder segments, but it looks rather weird and we can't exclude it's
actually causing problems.

Queue the empty ACK first, so that it gets a lower sequence number,
before checking for any data from the socket.

Reported-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

tcp.c

@@ -1957,11 +1957,12 @@ static void tcp_conn_from_sock_finish(const struct ctx *c,
 		return;
 	}
 
+	tcp_send_flag(c, conn, ACK);
+
 	/* The client might have sent data already, which we didn't
 	 * dequeue waiting for SYN,ACK from tap -- check now.
 	 */
 	tcp_data_from_sock(c, conn);
-	tcp_send_flag(c, conn, ACK);
 }
 
 /**