tcp: Add a new kernel-only TCP_USE_DDP socket option

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.

Reviewed by: rscheff, tuexen, glebius
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D44000