rust: debugfs: Use kernel Atomic type in docs example

    /// # Examples

    ///

    /// ```

    /// # use core::sync::atomic::{AtomicU32, Ordering};

    /// # use kernel::debugfs::Dir;

    /// # use kernel::prelude::*;

    /// # use kernel::{

    /// #     debugfs::Dir,

    /// #     prelude::*,

    /// #     sync::atomic::{

    /// #         Atomic,

    /// #         Relaxed,

    /// #     },

    /// # };

    /// # let dir = Dir::new(c"foo");

    /// let file = KBox::pin_init(

    ///     dir.read_callback_file(c"bar",

    ///     AtomicU32::new(3),

    ///     Atomic::<u32>::new(3),

    ///     &|val, f| {

    ///       let out = val.load(Ordering::Relaxed);

    ///       let out = val.load(Relaxed);

    ///       writeln!(f, "{out:#010x}")

    ///     }),

    ///     GFP_KERNEL)?;

    /// // Reading "foo/bar" will show "0x00000003".

    /// file.store(10, Ordering::Relaxed);

    /// file.store(10, Relaxed);

    /// // Reading "foo/bar" will now show "0x0000000a".

    /// # Ok::<(), Error>(())

    /// ```