tcp: bbr.c is non-capable of doing ECN and sets an INP flag to fend off ECN however our syncache is not aware of that flag.

[rrs]

Summary

We need to make the syncache aware of the flag and not do ECN if its set. Note that this
is not 100% full proof but the best we can do (i.e. its still possible that you can get in a
situation where the peer try's to do ecn).

Test Plan

Packet drill can be used to send in a syn with ecn and validate that sync cache
response won't have ECN enabled.