rust: clean Rust 1.88.0's unnecessary_transmutes lint

Starting with Rust 1.88.0 (expected 2025-06-26) [1][2], `rustc` may
introduce a new lint that catches unnecessary transmutes, e.g.:

     error: unnecessary transmute
         --> rust/uapi/uapi_generated.rs:23242:18
          |
    23242 |         unsafe { ::core::mem::transmute(self._bitfield_1.get(0usize, 1u8) as u8) }
          |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: replace this with: `(self._bitfield_1.get(0usize, 1u8) as u8 == 1)`
          |
          = note: `-D unnecessary-transmutes` implied by `-D warnings`
          = help: to override `-D warnings` add `#[allow(unnecessary_transmutes)]`

There are a lot of them (at least 300), but luckily they are all in
`bindgen`-generated code.

Thus clean all up by allowing it there.

Since unknown lints trigger a lint itself in older compilers, do it
conditionally so that we can keep the `unknown_lints` lint enabled.

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/pull/136083 [1]
Link: https://github.com/rust-lang/rust/issues/136067 [2]
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20250502140237.1659624-4-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

init/Kconfig

@@ -140,6 +140,9 @@ config LD_CAN_USE_KEEP_IN_OVERLAY
 config RUSTC_HAS_COERCE_POINTEE
 	def_bool RUSTC_VERSION >= 108400
 
+config RUSTC_HAS_UNNECESSARY_TRANSMUTES
+	def_bool RUSTC_VERSION >= 108800
+
 config PAHOLE_VERSION
 	int
 	default $(shell,$(srctree)/scripts/pahole-version.sh $(PAHOLE))

rust/bindings/lib.rs

@@ -26,6 +26,7 @@
 
 #[allow(dead_code)]
 #[allow(clippy::undocumented_unsafe_blocks)]
+#[cfg_attr(CONFIG_RUSTC_HAS_UNNECESSARY_TRANSMUTES, allow(unnecessary_transmutes))]
 mod bindings_raw {
     // Manual definition for blocklisted types.
     type __kernel_size_t = usize;

rust/uapi/lib.rs

@@ -24,6 +24,7 @@
     unreachable_pub,
     unsafe_op_in_unsafe_fn
 )]
+#![cfg_attr(CONFIG_RUSTC_HAS_UNNECESSARY_TRANSMUTES, allow(unnecessary_transmutes))]
 
 // Manual definition of blocklisted types.
 type __kernel_size_t = usize;