Imagine the following situation. Faceless Void having his stun skilled which stuns at a percentage rate and does extra damage. If he now buys a Battlefury and hits a direct target and stuns it, do the other targets who gets cleaved as well take some extra damage? If yes how many % ? 30% as the cleave says or is it the full extra damage from my skill ?
Actually, if you look at Faceless Void's Wiki, it says that the bash damage is magical.
Secondly, even if it were Slardar's bash, which is physical damage, Battle Fury would not cleave that damage. The only proc that adds to cleave damage is a critical hit. Your regular damage will be cleaved, but your bash damage will not.
You can read more about cleave here.