rust: init: change the generated name of guard variables

The initializers created by the `[try_][pin_]init!` macros utilize the
guard pattern to drop already initialized fields, when initialization
fails mid-way. These guards are generated to have the same name as the
field that they handle. To prevent namespacing issues [1] when the
field name is the same as e.g. a constant name, add `__` as a prefix
and `_guard` as the suffix.

[ Gary says:

   "Here's the simplified example:

    ```
    macro_rules! f {
        () => {
            let a = 1;
            let _: u32 = a;
        }
    }

    const a: u64 = 1;

    fn main() {
        f!();
    }
    ```

    The `a` in `f` have a different hygiene so normally it is scoped to the
    macro expansion and wouldn't escape. Interestingly a constant is still
    preferred despite the hygiene so constants escaped into the macro,
    leading to the error."

  - Miguel ]

Signed-off-by: Benno Lossin <benno.lossin@proton.me>
Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/rust-for-linux/1e8a2a1f-abbf-44ba-8344-705a9cbb1627@proton.me/ [1]
Link: https://lore.kernel.org/r/20240403194321.88716-1-benno.lossin@proton.me
[ Added Benno's link and Gary's simplified example. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

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