rust: sync: atomic: Add example for Atomic::get_mut()

    /// Returns a mutable reference to the underlying atomic `T`.

    ///

    /// This is safe because the mutable reference of the atomic `T` guarantees exclusive access.

    ///

    /// # Examples

    ///

    /// ```

    /// use kernel::sync::atomic::{Atomic, Relaxed};

    ///

    /// let mut atomic_val = Atomic::new(0u32);

    /// let val_mut = atomic_val.get_mut();

    /// *val_mut = 101;

    /// assert_eq!(101, atomic_val.load(Relaxed));

    /// ```

    pub fn get_mut(&mut self) -> &mut T {

        // CAST: `T` and `T::Repr` has the same size and alignment per the safety requirement of

        // `AtomicType`, and per the type invariants `self.0` is a valid `T`, therefore the casting