block/rnbd-srv: init ret with 0 instead of -EPERM

Let's always set errno after pr_err which is consistent with
default case.

Acked-by: Jack Wang <jinpu.wang@ionos.com>
Signed-off-by: Guoqing Jiang <guoqing.jiang@linux.dev>
Link: https://lore.kernel.org/r/20230524070026.2932-7-guoqing.jiang@linux.dev


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