tcp: Rename rfc6675_pipe to sack.revised, and enable by default

As full support of RFC6675 is in place, deprecating
net.inet.tcp.rfc6675_pipe and enabling by default
net.inet.tcp.sack.revised.

Reviewed By: transport, kbowling, rrs
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D28702