tcp: Make dsack stats be aware of TLPs

[rrs]

Summary

This changes the way we account for DSACK bytes with respect to
TLPs. We keep them as separate byte counts since a TLP that induces
a DSACK is somewhat expected versus ones that are not.

Question for Michael, right now this code is only if you have NETFLIX_STATS in place, but
maybe we might want to have some DSACK byte tracking stats. The reason we want it
is to compare against retransmissions and get a "feel" for if we are increasing or decreasing
needless retransmissions. In theory I could make the sysctl var's that track this in at least
the base tcp_subr..

Thoughts? Or for that matter maybe there is no need for this update :)

Test Plan

Make sure the counters are showing up.

[tuexen]

Maybe report it in TCP_INFO?

[rrs]

This refactors so that we end up with the data in netstat instead
so that it can be used by others besides NF. I also add the hooks
into tcp_sack so the counters are maintained for the base stack, though
we don't track TLPs (or do TLPs for that matter). BBR has the netstats
removed and now uses the proper api. Note that it does do TLP but
as of yet does not track what dsack bytes were caused by TLPs.