rack: Fix ECN on finalizing session.

Maintain code similarity between RACK and base stack
for ECN. This may not strictly be necessary, depending
when a state transition to FIN_WAIT_1 is done in RACK
after a shutdown() or close() syscall.

MFC after: 3 days
Reviewed By: tuexen, transport
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D29658