tcp: reserve tcp_info counters for AccECN

Marking all new fields unused (__xxx).

No functional change.

Reviewed By: tuexen, rrs, transport
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D37016

(cherry picked from commit 3708c3d370f26af0841cd3e97bf569d39c42953b)