btrfs: switch dev stats item to the permanent item key

Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/ctree.h

@@ -100,6 +100,9 @@ struct btrfs_ordered_sum;
 /* tracks free space in block groups. */
 #define BTRFS_FREE_SPACE_TREE_OBJECTID 10ULL
 
+/* device stats in the device tree */
+#define BTRFS_DEV_STATS_OBJECTID 0ULL
+
 /* for storing balance parameters in the root tree */
 #define BTRFS_BALANCE_OBJECTID -4ULL
 
@@ -2219,7 +2222,7 @@ struct btrfs_ioctl_defrag_range_args {
  *
  * - device statistics, store IO stats in the device tree, one key for all
  *   stats
- *   (0, BTRFS_DEV_STATS_KEY, 0)
+ *   (BTRFS_DEV_STATS_OBJECTID, BTRFS_DEV_STATS_KEY, 0)
  */
 #define BTRFS_PERSISTENT_ITEM_KEY	249
 

fs/btrfs/volumes.c

@@ -6705,8 +6705,8 @@ int btrfs_init_dev_stats(struct btrfs_fs_info *fs_info)
 		int item_size;
 		struct btrfs_dev_stats_item *ptr;
 
-		key.objectid = 0;
-		key.type = BTRFS_DEV_STATS_KEY;
+		key.objectid = BTRFS_DEV_STATS_OBJECTID;
+		key.type = BTRFS_PERSISTENT_ITEM_KEY;
 		key.offset = device->devid;
 		ret = btrfs_search_slot(NULL, dev_root, &key, path, 0, 0);
 		if (ret) {
@@ -6753,8 +6753,8 @@ static int update_dev_stat_item(struct btrfs_trans_handle *trans,
 	int ret;
 	int i;
 
-	key.objectid = 0;
-	key.type = BTRFS_DEV_STATS_KEY;
+	key.objectid = BTRFS_DEV_STATS_OBJECTID;
+	key.type = BTRFS_PERSISTENT_ITEM_KEY;
 	key.offset = device->devid;
 
 	path = btrfs_alloc_path();