block/rnbd-srv: no need to check sess_dev

Check ret is enough since if sess_dev is NULL which also
implies ret should be 0.

Acked-by: Jack Wang <jinpu.wang@ionos.com>
Signed-off-by: Guoqing Jiang <guoqing.jiang@linux.dev>
Link: https://lore.kernel.org/r/20230524070026.2932-5-guoqing.jiang@linux.dev


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