Optimize the TSO and copy paths to use the new tcp_m_copy routine

[rrs]

Summary

The TCP stack currently runs through the list of mbufs sizing
for TSO and then again for the actual copy out to the new
mbuf chain being passed down to IP. This causes double the
number of cache hits as we walk the mbuf chains.

For Rack a new tcp_m_copy() module was introduced that
optimizes this so the copy does both copy and size limiting
for TSO at the same time.

This change brings the use of that function into the main stack. Note
that NF has been using this in both rack and the main stack for a couple
of years now. This also paves the way for Drew Gallatin's new mbufs and
the TLS sendfile feature that uses those.

Test Plan

Bring it all up, which I have, and do some tcp connections
and network activity to make sure the stack works.

[gallatin]

I added Hans since it is mostly his code we are factoring out.

[kbowling]

I've deployed a variant of this widely at LLNW on the default stack as well. I think you can garbage collect sbsndptr() from the tree.

[gallatin]

In D15937#337221, @kbowling wrote:

I've deployed a variant of this widely at LLNW on the default stack as well. I think you can garbage collect sbsndptr() from the tree.

The cxgbe TOE stuff uses it

[• hselasky]

Looks good to me.