As a TCP client only enable ECN when the corresponding sysctl variable

indicates that ECN should be negotiated for the client side.

Submitted by:		Richard Scheffenegger
Reviewed by:		rgrimes@, tuexen@
MFC after:		1 week
Differential Revision:	https://reviews.freebsd.org/D23228

sys/netinet/tcp_input.c

@@ -1979,7 +1979,7 @@ tcp_do_segment(struct mbuf *m, struct tcphdr *th, struct socket *so,
 				tp->t_flags |= TF_ACKNOW;
 
 			if (((thflags & (TH_CWR | TH_ECE)) == TH_ECE) &&
-			    V_tcp_do_ecn) {
+			    (V_tcp_do_ecn == 1)) {
 				tp->t_flags2 |= TF2_ECN_PERMIT;
 				TCPSTAT_INC(tcps_ecn_shs);
 			}

sys/netinet/tcp_stacks/rack.c

@@ -6296,7 +6296,7 @@ rack_do_syn_sent(struct mbuf *m, struct tcphdr *th, struct socket *so,
 		}
 
 		if (((thflags & (TH_CWR | TH_ECE)) == TH_ECE) &&
-		    V_tcp_do_ecn) {
+		    (V_tcp_do_ecn == 1)) {
 			tp->t_flags2 |= TF2_ECN_PERMIT;
 			TCPSTAT_INC(tcps_ecn_shs);
 		}