This patch adds SMCv2.x supplemental features negotiation. Supported
SMCv2.x supplemental features are represented by feature_mask in FCE
field. The negotiation process is as follows.

 Server                                        Client
            Proposal(features(c-mask bits))
      <-----------------------------------------
            Accept(features(s-mask bits))
      ----------------------------------------->
           Confirm(features(s&c-mask bits))
      <-----------------------------------------
Signed-off-by: Wen Gu <guwen@linux.alibaba.com>
Reviewed-and-tested-by: Wenjia Zhang <wenjia@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>