read/write volatile

Note that the kernel unconditionally provide 64-bit atomic operation to all archs, and in archs that doesn't support native atomics on 64-bit integers, they are implemented using locks. This means that using `READ_ONCE()` on u64 for atomic ops is incorrect (it needs to be a `atomic64_read()`).

`READ_ONCE` on u64 might still be useful is you just want to read the value in a data-race-free way and you don't care about atomicity (i.e. allow the read to tear). However this is yet another reason I don't want people to just use `READ_ONCE()` for atomic ops on Rust side -- it's just not intuitive which semantics is desired.