tcp: enable LRD by default

Lost Retransmission Detection was added as a
feature in May 2021, but disabled by default.

Enabling the feature by default to reduce the
flow completion time by avoiding RTOs when
retransmissions get lost too.

Reviewed By: tuexen, transport, zlei
MFC after: 10 weeks
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D42845