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

This header describes main structs and functions used by rnbd-client
module, mainly for managing RNBD sessions and mapped block devices,
creating and destroying sysfs entries.

Link: https://lore.kernel.org/r/20200511135131.27580-17-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>