tcp cc: re-organize newreno functions into parts that can be re-used

[cc]

Summary

Prepare this patch for testing in 14/stable.

[cc]

Does not seem to be needed.

[rrs]

What other CC modules are you contemplating using this for?

My one comment on the change is that now you will always be setting something into the cwnd.. the old code only
did it when there was a change. Have not looked at the cache line layout of the data, I suspect it makes no difference
since you probably have the cwnd in cache anyway :)

[cc]

In D47218#1253522, @rrs wrote:

What other CC modules are you contemplating using this for?

My one comment on the change is that now you will always be setting something into the cwnd.. the old code only
did it when there was a change. Have not looked at the cache line layout of the data, I suspect it makes no difference
since you probably have the cwnd in cache anyway :)

Thanks for the cache check!

This patch and another patch D47213 were used to backport commit b6c137de0af1a2bbd4c73bbc9cd96325268f3b0e and commit ee45061051715be4704ba22d2fcd1c373e29079d to see if tested, a user could have throughput improvement from VMs in FreeBSD14-stable. The subject is https://lists.freebsd.org/archives/freebsd-net/2024-October/005851.html.

But the user found out the bottleneck was somewhereelse and so abandon this backport effort. :)