net/smc: add SMC-D support in data transfer

net/smc: add SMC-D support in data transfer

The data transfer and CDC message headers differ in SMC-R and SMC-D.
This patch adds support for the SMC-D data transfer to the existing SMC
code. It consists of the following:

* SMC-D CDC support
* SMC-D tx support
* SMC-D rx support

The CDC header is stored at the beginning of the receive buffer. Thus, a
rx_offset variable is added for the CDC header offset within the buffer
(0 for SMC-R).

Signed-off-by: Hans Wippel <[email protected]>
Signed-off-by: Ursula Braun <[email protected]>
Suggested-by: Thomas Richter <[email protected]>
Signed-off-by: David S. Miller <[email protected]>