block/rnbd-srv: init err earlier in rnbd_srv_init_module

With this, we can remove several lines of code.

Acked-by: Jack Wang <jinpu.wang@ionos.com>
Signed-off-by: Guoqing Jiang <guoqing.jiang@linux.dev>
Link: https://lore.kernel.org/r/20230524070026.2932-8-guoqing.jiang@linux.dev


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