btrfs: fix btrfs_extend_item() comment

The size parameter to btrfs_extend_item() is the number of bytes
to add to the item, not the size of the item after the operation
(like it is for btrfs_truncate_item(), there the size parameter
is not the number of bytes to take away, but the total size of
the item after truncation).
Fix it in the comment.
Signed-off-by: NStefan Behrens <sbehrens@giantdisaster.de>
Signed-off-by: NJiri Kosina <jkosina@suse.cz>

fs/btrfs/ctree.c

@@ -4430,7 +4430,7 @@ void btrfs_truncate_item(struct btrfs_root *root, struct btrfs_path *path,
 }
 
 /*
- * make the item pointed to by the path bigger, data_size is the new size.
+ * make the item pointed to by the path bigger, data_size is the added size.
  */
 void btrfs_extend_item(struct btrfs_root *root, struct btrfs_path *path,
 		       u32 data_size)