tcp: Correct handling of FIN,ACK followed by SYN

When the guest tries to establish a connection, it could give up on it by
sending a FIN,ACK instead of a plain ACK to our SYN,ACK.  It could then
make a new attempt to establish a connection with the same addresses and
ports with a new SYN.

Although it's unlikely, it could send the 2nd SYN very shortly after the
FIN,ACK resulting in both being received in the same batch of packets from
the tap interface.

Currently, we don't handle that correctly, when we receive a FIN,ACK on a
not fully established connection we discard the remaining packets in the
batch, and so will never process the 2nd SYN.  Correct this by returning
1 from tcp_tap_handler() in this case, so we'll just consume the FIN,ACK
and continue to process the rest of the batch.

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

tcp.c

@@ -2598,7 +2598,7 @@ int tcp_tap_handler(struct ctx *c, int af, const void *saddr, const void *daddr,
 			tcp_send_flag(c, conn, ACK);
 			conn_event(c, conn, SOCK_FIN_SENT);
 
-			return p->count - idx;
+			return 1;
 		}
 
 		if (!th->ack)