tcp: Add a new kernel-only TCP_USE_DDP socket option

[jhb]

Summary

This socket option can be used by in-kernel consumers (like NFS) to
request a NIC to use optimized receive of large buffers for a
connection. The current use case is to support DDP by the TOE on
Chelsio NICs.

Sponsored by: Chelsio Communications

[jhb]

I also considered adding this as a protocol-independent socket option (SO_DDP) but that is somewhat annoying when it comes to implementation in the next patch in the series as the TOE module only really gets to hook pr_ctloutput and socket-level options don't invoke that hook normally.

[glebius]

btw, I'm getting close to divorce unix(4) from soreceive_generic/sosend_generic. That means the functions are left for TCP and a few leftovers (rtsock, bluetooth). The plan to create tcp_soreceive() and tcp_sosend() that will not carry any code for other sockets and would be much easier to extend.