tcp: Restore correct ECT marking behavior on SACK retransmissions

While coalescing all ECN-related code into new common source files,
the flag to deal with SACK retransmissions was skipped. This leads
to non-compliant ECT-marking of SACK retransmissions, as well as
the premature sending of other TCP ECN flags (CWR).

Reviewed By: rrs, transport
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D34376