Since we are starting to support several Rust toolchains, lints (including
Clippy ones) now may behave differently and lint groups may include
new lints.

Therefore, to maximize the chances a given version works, relax some
deny-level lints to warnings. It may also make our lives a bit easier
while developing new code or refactoring.

To be clear, the requirements for in-tree code are still the same, since
Rust code still needs to be warning-free (patches should be clean under
`WERROR=y`) and the set of lints is not changed.

`unsafe_op_in_unsafe_fn` is left unmodified, i.e. as an error, since it is
becoming the default in the language (warn-by-default in Rust 2024 [1] and
ideally an error later on) and thus it should also be very well tested. In
addition, it is simple enough that it should not have false positives
(unlike e.g. `rust_2018_idioms`'s `explicit_outlives_requirements`).

`non_ascii_idents` is left unmodified as well, i.e. as an error, since
it is unlikely one gains any productivity during development if it
were a warning (in fact, it may be worse, since it is likely one made
a typo). In addition, it should not have false positives.

Finally, put the two `-D` ones at the top and take the chance to do one
per line.

Link: https://github.com/rust-lang/rust/pull/112038 [1]
Reviewed-by: Finn Behrens <me@kloenk.dev>
Tested-by: Benno Lossin <benno.lossin@proton.me>
Tested-by: Andreas Hindborg <a.hindborg@samsung.com>
Link: https://lore.kernel.org/r/20240709160615.998336-5-ojeda@kernel.orgSigned-off-by: Miguel Ojeda <ojeda@kernel.org>