rnbd: add error handling support for add_disk()

We never checked for errors on add_disk() as this function
returned void. Now that this is fixed, use the shiny new
error handling.

Acked-by: Jack Wang <jinpu.wang@ionos.com>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
Link: https://lore.kernel.org/r/20211015233028.2167651-9-mcgrof@kernel.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>