Support for TCP DDP (Direct Data Placement) in the T4 TOE module.

Basically, this is automatic rx zero copy when feasible. TCP payload is
DMA'd directly into the userspace buffer described by the uio submitted
in soreceive by an application.

• Works with sockets that are being handled by the TCP offload engine of a T4 chip (you need t4_tom.ko module loaded after cxgbe, and an "ifconfig +toe" on the cxgbe interface).
• Does not require any modification to the application.
• Not enabled by default. Use hw.t4nex.<X>.toe.ddp="1" to enable it.