Current use of `pin` is unsound

[nbdd0121]

In rust_miscdev, this would compile:

let mut state = SharedState::try_new()?;
let mut state2 = SharedState::try_new()?;
core::mem::swap(Arc::get_mut(&mut state).unwrap(), Arc::get_mut(&mut state2).unwrap()); 

but this swaps two pinned CondVar, which is unsound. After pinning the CondVar, we shouldn't be able to obtain any Arc<SharedState>, but rather only Pin<Arc<SharedState>>.

[alex]

The right fix here is:

a) To pin the Arc immediately here: https://github.com/Rust-for-Linux/linux/blob/rust/samples/rust/rust_miscdev.rs#L42
b) To change teh return type to Pin<Arc<SharedState>> here: https://github.com/Rust-for-Linux/linux/blob/rust/samples/rust/rust_miscdev.rs#L41
c) To update anything else to get it to compile :-)