[tcp] Fix ECN on finalizing sessions.

A subtle oversight would subtly change new data packets
sent after a shutdown() or close() call, while the send
buffer is still draining.

MFC after: 3 days
Reviewed By: transport, tuexen
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D29616

(cherry picked from commit 9f2eeb02623d6a847a90da68a5892c25b14ce2d4)