rust: cpumask: Replace `MaybeUninit` and `mem::zeroed` with `Opaque` APIs

Replace the following unsafe initializations:
1. `MaybeUninit::uninit().assume_init()` with `Opaque::uninit()`
2. `core::mem::zeroed()` with `Opaque::zeroed()`

Suggested-by: Benno Lossin <lossin@kernel.org>
Link: https://github.com/Rust-for-Linux/linux/issues/1178


Suggested-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/rust-for-linux/CAH5fLgj0OoCn56OkNUmiPQ=RAVa_VmS-yMZ4TNBSpGPNtZ5D0A@mail.gmail.com/


Reviewed-by: Benno Lossin <lossin@kernel.org>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Ritvik Gupta <ritvikfoss@gmail.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>