block/rnbd: private headers with rnbd protocol structs and helpers

These are common private headers with rnbd protocol structures, logging,
sysfs and other helper functions, which are used on both client and server
sides.

Link: https://lore.kernel.org/r/20200511135131.27580-16-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>