rust: allow clippy::as_underscore in the generated bindings

A CLIPPY=1 build emitted about 15000 `as _` conversion warnings, all of
them in bindgen's generated output and none in hand-written code.

[ The lint messages look like:

    error: using `as _` conversion
         --> rust/bindings/bindings_generated.rs:18947:9
          |
    18947 |         self._bitfield_1.get_const::<0usize, 16u8>() as u32 as _
          |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^-
          |                                                                |
          |                                                                help: consider giving the type explicitly: `u32`
          |
          = help: for further information visit https://rust-lang.github.io/rust-clippy/rust-1.98.0/index.html#as_underscore
          = note: `-D clippy::as-underscore` implied by `-D warnings`
          = help: to override `-D warnings` add `#[allow(clippy::as_underscore)]`

    - Miguel ]

bindgen 0.73 returns each bitfield read through a trailing `as _`, and
0.72 returns it through a transmute, which the lint ignores. The
bindings and uapi crates allow `clippy::all` over the generated code.
That group does not cover `clippy::as_underscore`, a restriction lint.

Allow `clippy::as_underscore` by name in the bindings and uapi crates.

Assisted-by: LLM
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Link: https://patch.msgid.link/20260906215822.1201022-1-jhubbard@nvidia.com
Cc: stable@vger.kernel.org # Needed in 6.12.y and later (Rust is pinned in older LTSs).
[ Removed CI sentence. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/bindings/lib.rs

@@ -22,6 +22,7 @@
 #![feature(cfi_encoding)]
 
 #[allow(dead_code)]
+#[allow(clippy::as_underscore)]
 #[allow(clippy::cast_lossless)]
 #[allow(clippy::ptr_as_ptr)]
 #[allow(clippy::ref_as_ptr)]

rust/uapi/lib.rs

@@ -10,6 +10,7 @@
 #![no_std]
 #![allow(
     clippy::all,
+    clippy::as_underscore,
     clippy::cast_lossless,
     clippy::ptr_as_ptr,
     clippy::ref_as_ptr,