Clamp the initial advertised receive window when responding to a SYN/ACK

to the maximum allowed window.  Growing the window too large would cause
an underflow in the calculations in tcp_output() to decide if a window
update should be sent which would prevent the persist timer from being
started if data was pending and the other end of the connection advertised
an initial window size of 0.

PR:		kern/154006
Submitted by:	Stefan `Sec` Zehl  sec 42 org
Reviewed by:	bz
MFC after:	1 week

sys/netinet/tcp_input.c

@@ -1756,7 +1756,8 @@ tcp_do_segment(struct mbuf *m, struct tcphdr *th, struct socket *so,
 				(TF_RCVD_SCALE|TF_REQ_SCALE)) {
 				tp->rcv_scale = tp->request_r_scale;
 			}
-			tp->rcv_adv += tp->rcv_wnd;
+			tp->rcv_adv += imin(tp->rcv_wnd,
+			    TCP_MAXWIN << tp->rcv_scale);
 			tp->snd_una++;		/* SYN is acked */
 			/*
 			 * If there's data, delay ACK; if there's also a FIN