rds: make v3.1 as compat version

Mark RDSv3.1 as compat version and add v4.1 version macro's.
Subsequent patches enable TOS(Type of Service) feature which is
tied with v4.1 for RDMA transport.

Reviewed-by: Sowmini Varadhan <sowmini.varadhan@oracle.com>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>
[yanjun.zhu@oracle.com: Adapted original patch with ipv6 changes]
Signed-off-by: Zhu Yanjun <yanjun.zhu@oracle.com>