rbd: use rbd_obj_method_sync() return value

Now that rbd_obj_method_sync() returns the number of bytes
returned by the method call, that value should be used by
callers to ensure we don't overrun the valid portion of the
buffer.

Fix the two spots that remained that weren't doing that,
rbd_dev_image_name() and rbd_dev_v2_snap_name().

Rearrange the error path slightly in rbd_dev_v2_snap_name().
Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>

drivers/block/rbd.c

@@ -2614,7 +2614,8 @@ static int rbd_dev_header_watch_sync(struct rbd_device *rbd_dev, int start)
 }
 
 /*
- * Synchronous osd object method call
+ * Synchronous osd object method call.  Returns the number of bytes
+ * returned in the outbound buffer, or a negative error code.
  */
 static int rbd_obj_method_sync(struct rbd_device *rbd_dev,
 			     const char *object_name,
@@ -3741,7 +3742,8 @@ static char *rbd_dev_image_name(struct rbd_device *rbd_dev)
 	if (ret < 0)
 		goto out;
 	p = reply_buf;
-	end = reply_buf + size;
+	end = reply_buf + ret;
+
 	image_name = ceph_extract_encoded_string(&p, end, &len, GFP_KERNEL);
 	if (IS_ERR(image_name))
 		image_name = NULL;
@@ -3914,26 +3916,23 @@ static char *rbd_dev_v2_snap_name(struct rbd_device *rbd_dev, u32 which)
 				&snap_id, sizeof (snap_id),
 				reply_buf, size, NULL);
 	dout("%s: rbd_obj_method_sync returned %d\n", __func__, ret);
-	if (ret < 0)
+	if (ret < 0) {
+		snap_name = ERR_PTR(ret);
 		goto out;
+	}
 
 	p = reply_buf;
-	end = reply_buf + size;
+	end = reply_buf + ret;
 	snap_name = ceph_extract_encoded_string(&p, end, NULL, GFP_KERNEL);
-	if (IS_ERR(snap_name)) {
-		ret = PTR_ERR(snap_name);
+	if (IS_ERR(snap_name))
 		goto out;
-	} else {
-		dout("  snap_id 0x%016llx snap_name = %s\n",
-			(unsigned long long)le64_to_cpu(snap_id), snap_name);
-	}
-	kfree(reply_buf);
 
-	return snap_name;
+	dout("  snap_id 0x%016llx snap_name = %s\n",
+		(unsigned long long)le64_to_cpu(snap_id), snap_name);
 out:
 	kfree(reply_buf);
 
-	return ERR_PTR(ret);
+	return snap_name;
 }
 
 static char *rbd_dev_v2_snap_info(struct rbd_device *rbd_dev, u32 which,