Documentation: RDS: updates for SO_RDS_TRANSPORT socket option

  bind(fd, &sockaddr_in, ...)

        This binds the socket to a local IP address and port, and a

        transport.

        transport, if one has not already been selected via the

	SO_RDS_TRANSPORT socket option

  sendmsg(fd, ...)

        Sends a message to the indicated recipient. The kernel will

        operation. In this case, it would use RDS_CANCEL_SENT_TO to

        nuke any pending messages.

  setsockopt(fd, SOL_RDS, SO_RDS_TRANSPORT, (int *)&transport ..)

  getsockopt(fd, SOL_RDS, SO_RDS_TRANSPORT, (int *)&transport ..)

	Set or read an integer defining  the underlying

	encapsulating transport to be used for RDS packets on the

	socket. When setting the option, integer argument may be

	one of RDS_TRANS_TCP or RDS_TRANS_IB. When retrieving the

	value, RDS_TRANS_NONE will be returned on an unbound socket.

	This socket option may only be set exactly once on the socket,

	prior to binding it via the bind(2) system call. Attempts to

	set SO_RDS_TRANSPORT on a socket for which the transport has

	been previously attached explicitly (by SO_RDS_TRANSPORT) or

	implicitly (via bind(2)) will return an error of EOPNOTSUPP.

	An attempt to set SO_RDS_TRANSPPORT to RDS_TRANS_NONE will

	always return EINVAL.

RDMA for RDS

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