rbd: treat images mapped read-only seriously

Even though -o ro/-o read_only/--read-only options are very old, we
have never really treated them seriously (on par with snapshots).  As
a first step, fail writes to images mapped read-only just like we do
for snapshots.

We need this check in rbd because the block layer basically ignores
read-only setting, see commit a32e236eb9 ("Partially revert "block:
fail op_is_write() requests to read-only partitions"").

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Jason Dillaman <dillaman@redhat.com>
Reviewed-by: Dongsheng Yang <dongsheng.yang@easystack.cn>

drivers/block/rbd.c

@@ -4837,11 +4837,14 @@ static void rbd_queue_workfn(struct work_struct *work)
 		goto err_rq;
 	}
 
-	if (op_type != OBJ_OP_READ && rbd_is_snap(rbd_dev)) {
-		rbd_warn(rbd_dev, "%s on read-only snapshot",
-			 obj_op_name(op_type));
-		result = -EIO;
-		goto err;
+	if (op_type != OBJ_OP_READ) {
+		if (rbd_is_ro(rbd_dev)) {
+			rbd_warn(rbd_dev, "%s on read-only mapping",
+				 obj_op_name(op_type));
+			result = -EIO;
+			goto err;
+		}
+		rbd_assert(!rbd_is_snap(rbd_dev));
 	}
 
 	/*