rnbd-srv: remove struct rnbd_dev

Given that rnbd_srv_sess_dev already has an open_flags member, there
is no need for the rnbd_dev indirection as a simple block_device pointer
works just as well.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Acked-by: Jack Wang <jinpu.wang@ionos.com>
Link: https://lore.kernel.org/r/20220909131509.3263924-5-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>