rnbd-srv: add comment in rnbd_srv_rdma_ev

Let's add some explanations here given the err handling is not obvious.

Signed-off-by: Guoqing Jiang <guoqing.jiang@linux.dev>
Acked-by: Md Haris Iqbal <haris.iqbal@ionos.com>
Link: https://lore.kernel.org/r/20220902100055.25724-2-guoqing.jiang@linux.dev
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/rnbd/rnbd-srv.c

@@ -402,6 +402,11 @@ static int rnbd_srv_rdma_ev(void *priv,
 		return -EINVAL;
 	}
 
+	/*
+	 * Since ret is passed to rtrs to handle the failure case, we
+	 * just return 0 at the end otherwise callers in rtrs would call
+	 * send_io_resp_imm again to print redundant err message.
+	 */
 	rtrs_srv_resp_rdma(id, ret);
 	return 0;
 }