RDMA/rtrs: client: private header with client structs and functions

This header describes main structs and functions used by rtrs-client
module, mainly for managing rtrs sessions, creating/destroying sysfs
entries, accounting statistics on client side.

Link: https://lore.kernel.org/r/20200511135131.27580-6-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>