tcp: use tcp_fixed_maxseg instead of tcp_maxseg in cc modules

tcp_fixed_maxseg() is the streamlined calculation of typical
tcp options and more suitable for heavy use in the congestion
control modules on every received packet.

No external functional change.

Reviewed By:           tuexen, #transport
Sponsored by:          NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D43779

sys/netinet/cc/cc.c

@@ -440,7 +440,7 @@ newreno_cc_after_idle(struct cc_var *ccv)
 	 * maximum of the former ssthresh or 3/4 of the old cwnd, to
 	 * not exit slow-start prematurely.
 	 */
-	rw = tcp_compute_initwnd(tcp_maxseg(ccv->ccvc.tcp));
+	rw = tcp_compute_initwnd(tcp_fixed_maxseg(ccv->ccvc.tcp));
 
 	CCV(ccv, snd_ssthresh) = max(CCV(ccv, snd_ssthresh),
 	    CCV(ccv, snd_cwnd)-(CCV(ccv, snd_cwnd)>>2));

sys/netinet/cc/cc_cubic.c

@@ -423,7 +423,7 @@ cubic_cong_signal(struct cc_var *ccv, uint32_t type)
 	u_int mss;
 
 	cubic_data = ccv->cc_data;
-	mss = tcp_maxseg(ccv->ccvc.tcp);
+	mss = tcp_fixed_maxseg(ccv->ccvc.tcp);
 
 	switch (type) {
 	case CC_NDUPACK:

sys/netinet/cc/cc_dctcp.c

@@ -245,7 +245,7 @@ dctcp_cong_signal(struct cc_var *ccv, uint32_t type)
 	if (CCV(ccv, t_flags2) & TF2_ECN_PERMIT) {
 		dctcp_data = ccv->cc_data;
 		cwin = CCV(ccv, snd_cwnd);
-		mss = tcp_maxseg(ccv->ccvc.tcp);
+		mss = tcp_fixed_maxseg(ccv->ccvc.tcp);
 
 		switch (type) {
 		case CC_NDUPACK:

sys/netinet/cc/cc_htcp.c

@@ -284,7 +284,7 @@ htcp_cong_signal(struct cc_var *ccv, uint32_t type)
 	u_int mss;
 
 	htcp_data = ccv->cc_data;
-	mss = tcp_maxseg(ccv->ccvc.tcp);
+	mss = tcp_fixed_maxseg(ccv->ccvc.tcp);
 
 	switch (type) {
 	case CC_NDUPACK: