rbd: be more informative on -ENOENT failures

pr_info what exactly was the culprit: missing pool, image or snap.
Signed-off-by: NIlya Dryomov <idryomov@gmail.com>

drivers/block/rbd.c

@@ -5301,8 +5301,13 @@ static int rbd_dev_image_probe(struct rbd_device *rbd_dev, bool mapping)
 
 	if (mapping) {
 		ret = rbd_dev_header_watch_sync(rbd_dev);
-		if (ret)
+		if (ret) {
+			if (ret == -ENOENT)
+				pr_info("image %s/%s does not exist\n",
+					rbd_dev->spec->pool_name,
+					rbd_dev->spec->image_name);
 			goto out_header_name;
+		}
 	}
 
 	ret = rbd_dev_header_info(rbd_dev);
@@ -5319,8 +5324,14 @@ static int rbd_dev_image_probe(struct rbd_device *rbd_dev, bool mapping)
 		ret = rbd_spec_fill_snap_id(rbd_dev);
 	else
 		ret = rbd_spec_fill_names(rbd_dev);
-	if (ret)
+	if (ret) {
+		if (ret == -ENOENT)
+			pr_info("snap %s/%s@%s does not exist\n",
+				rbd_dev->spec->pool_name,
+				rbd_dev->spec->image_name,
+				rbd_dev->spec->snap_name);
 		goto err_out_probe;
+	}
 
 	if (rbd_dev->header.features & RBD_FEATURE_LAYERING) {
 		ret = rbd_dev_v2_parent_info(rbd_dev);
@@ -5390,8 +5401,11 @@ static ssize_t do_rbd_add(struct bus_type *bus,
 
 	/* pick the pool */
 	rc = rbd_add_get_pool_id(rbdc, spec->pool_name);
-	if (rc < 0)
+	if (rc < 0) {
+		if (rc == -ENOENT)
+			pr_info("pool %s does not exist\n", spec->pool_name);
 		goto err_out_client;
+	}
 	spec->pool_id = (u64)rc;
 
 	/* The ceph file layout needs to fit pool id in 32 bits */