RDMA/rtrs: public interface header to establish RDMA connections

Introduce public header which provides set of API functions to establish
RDMA connections from client to server machine using RTRS protocol, which
manages RDMA connections for each session, does multipathing and load
balancing.

Main functions for client (active) side:

 rtrs_clt_open() - Creates set of RDMA connections incapsulated
                    in IBTRS session and returns pointer on RTRS
		    session object.
 rtrs_clt_close() - Closes RDMA connections associated with RTRS
                     session.
 rtrs_clt_request() - Requests zero-copy RDMA transfer to/from
                       server.

Main functions for server (passive) side:

 rtrs_srv_open() - Starts listening for RTRS clients on specified
                    port and invokes RTRS callbacks for incoming
		    RDMA requests or link events.
 rtrs_srv_close() - Closes RTRS server context.

Link: https://lore.kernel.org/r/20200511135131.27580-3-danil.kipnis@cloud.ionos.com


Signed-off-by: Danil Kipnis <danil.kipnis@cloud.ionos.com>
Signed-off-by: Jack Wang <jinpu.wang@cloud.ionos.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>