RDMA/rtrs: core: lib functions shared between client and server modules

This is a set of library functions existing as a rtrs-core module, used by
client and server modules.

Mainly these functions wrap IB and RDMA calls and provide a bit higher
abstraction for implementing of RTRS protocol on client or server sides.

Link: https://lore.kernel.org/r/20200511135131.27580-5-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>