block/rnbd: server: private header with server structs and functions

This header describes main structs and functions used by rnbd-server
module, namely structs for managing sessions from different clients and
mapped (opened) devices.

Link: https://lore.kernel.org/r/20200511135131.27580-20-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>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Acked-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>