The number of distinct key types is not that big that we could waste one
for something new we want to store in the tree. We'll introduce a new
name for an existing key value and use the objectid for further
extension.  The victim is the BTRFS_BALANCE_ITEM_KEY (248).

The nature of the balance status item is a good example of the temporary
item. It exists from beginning of the balance, keeps the status until it
finishes.
Signed-off-by: NDavid Sterba <dsterba@suse.com>