An InlineExtent can be modified without locking the Tree

[asomers]

Tree::get is supposed to return by value, not by reference. But InlineExtent (and ExtAttr::Inline) values contain an embedded Arc<DivBufShared>. So if calling code, for example in Fs::do_truncate, gets such a value via Tree::get, it can mutate the Arc-protected buffer that is still owned by the Tree. The result is a mutation of an unlocked Tree, which is not good.

Rust's type system should be used to prevent this unlocked mutation, perhaps by returning something more like an Arc<[u8]>. Or, perhaps Tree::get should refuse to return such a value, and there should be a method like Tree::with that can access the Value while still holding the Tree's leaf node locked.