ECN incorrectly set

[rrs]

Summary

ECN requires that on the SYN both the CWR and ECT bits be set in order
for it to be considered a "request" for ECN. We therefore did not
correctly check for this condition. This fix should make it so we
enforce the RFC properly.

Test Plan

We can use pkt-drill to send in a SYN and make sure that it only
responds to the correct "ECN condtion" i.e. both bits set.

[rscheff]

Good catch!

I should have found that in D23230, when extending the meaning of V_tcp_do_ecn (for ECN+ and ECN++).

Please have a look into D23373 (simultaneous opens) also.