rust: sync: implement `Default` for `LockClassKey`

In the upcoming Rust 1.78.0, Clippy suggests to implement `Default` even
when `new()` is `const`, since `Default::default()` may call `const`
functions even if it is not `const` itself [1]:

    error: you should consider adding a `Default` implementation for `LockClassKey`
      --> rust/kernel/sync.rs:31:5
       |
    31 | /     pub const fn new() -> Self {
    32 | |         Self(Opaque::uninit())
    33 | |     }
       | |_____^

Thus implement it.

Link: https://github.com/rust-lang/rust-clippy/pull/10903

 [1]
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
Link: https://lore.kernel.org/r/20240401212303.537355-2-ojeda@kernel.org


Signed-off-by: Miguel Ojeda <ojeda@kernel.org>