tcp: add gone_in note for net.inet.tcp.sack.revised for fbsd16

Depricate the support for the old RFC3517 behavior of SACK loss
recovery, and simplfy the code to always adhere to RFC6675.

Reviewed By: tuexen, cc, transport
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D52383