btrfs: reduce arguments of helpers space accounting root item

There are two helpers to increase used bytes of root items that add or
subtract one node size, we don't need to pass the argument for that.
Rename the function so it matches the root item member that gets
changed.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>