rust: clean Rust 1.88.0's warning about `clippy::disallowed_macros` configuration

rust: clean Rust 1.88.0's warning about `clippy::disallowed_macros` configuration

Starting with Rust 1.88.0 (expected 2025-06-26) [1], Clippy may start
warning about paths that do not resolve in the `disallowed_macros`
configuration:

    warning: `kernel::dbg` does not refer to an existing macro
      --> .clippy.toml:10:5
       |
    10 |     { path = "kernel::dbg", reason = "the `dbg!` macro is intended as a debugging tool" },
       |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

This is a lint we requested at [2], due to the trouble debugging
the lint due to false negatives (e.g. [3]), which we use to emulate
`clippy::dbg_macro` [4]. See commit 8577c9dca799 ("rust: replace
`clippy::dbg_macro` with `disallowed_macros`") for more details.

Given the false negatives are not resolved yet, it is expected that
Clippy complains about not finding this macro.

Thus, until the false negatives are fixed (and, even then, probably we
will need to wait for the MSRV to raise enough), use the escape hatch
to allow an invalid path.

Cc: stable@vger.kernel.org # Needed in 6.12.y and later (Rust is pinned in older LTSs).
Link: https://github.com/rust-lang/rust-clippy/pull/14397
Link: https://github.com/rust-lang/rust-clippy/issues/11432
Link: https://github.com/rust-lang/rust-clippy/issues/11431
Link: https://github.com/rust-lang/rust-clippy/issues/11303
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20250502140237.1659624-5-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>