[PATCH 6.12.y] rust: block: `allow(deprecated)` for `fetch_update` for Rust >= 1.99.0

[Miguel Ojeda <ojeda@kernel.org>]

Starting with Rust 1.99.0 (expected 2026-10-01), the
`Atomic*::fetch_update` method is deprecated, with the compiler suggesting
the `try_update` alias instead:

    error: use of deprecated method `core::sync::atomic::Atomic::<u64>::fetch_update`: renamed to `try_update` for consistency
       --> rust/kernel/block/mq/request.rs:206:22
        |
    206 |     let old = target.fetch_update(Ordering::Relaxed, Ordering::Relaxed, |x| Some(op(x)));
        |                      ^^^^^^^^^^^^
        |
        = note: `-D deprecated` implied by `-D warnings`
        = help: to override `-D warnings` add `#[allow(deprecated)]`
    help: replace the use of the deprecated method
        |
    206 -     let old = target.fetch_update(Ordering::Relaxed, Ordering::Relaxed, |x| Some(op(x)));
    206 +     let old = target.try_update(Ordering::Relaxed, Ordering::Relaxed, |x| Some(op(x)));
        |

The deprecation was added in Rust 1.95.0 [1], but only triggers starting
with Rust 1.99.0.

However, we cannot use the alias since our minimum in 6.12.y is Rust
1.78.0 -- `try_update` was added in Rust 1.86.0 [2].

Thus just allow the lint.

Cc: Andreas Hindborg <a.hindborg@kernel.org>
Cc: Boqun Feng <boqun@kernel.org>
Cc: Jens Axboe <axboe@kernel.dk>
Link: https://github.com/rust-lang/rust/pull/148590 [1]
Link: https://github.com/rust-lang/rust/pull/133829 [2]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
---
Warning: this is a 6.12.y-only patch -- I assume you prefer something
local like this instead of backporting the commit that made this go away
later with the move to `Refcount`.

 rust/kernel/block/mq/request.rs | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/rust/kernel/block/mq/request.rs b/rust/kernel/block/mq/request.rs
index 7943f43b9575..1c7937653542 100644
--- a/rust/kernel/block/mq/request.rs
+++ b/rust/kernel/block/mq/request.rs
@@ -203,6 +203,7 @@ unsafe impl<T: Operations> Sync for Request<T> {}
 /// Store the result of `op(target.load())` in target, returning new value of
 /// target.
 fn atomic_relaxed_op_return(target: &AtomicU64, op: impl Fn(u64) -> u64) -> u64 {
+    #[allow(deprecated)]
     let old = target.fetch_update(Ordering::Relaxed, Ordering::Relaxed, |x| Some(op(x)));

     // SAFETY: Because the operation passed to `fetch_update` above always
@@ -215,6 +216,7 @@ fn atomic_relaxed_op_return(target: &AtomicU64, op: impl Fn(u64) -> u64) -> u64
 /// Store the result of `op(target.load)` in `target` if `target.load() !=
 /// pred`, returning [`true`] if the target was updated.
 fn atomic_relaxed_op_unless(target: &AtomicU64, op: impl Fn(u64) -> u64, pred: u64) -> bool {
+    #[allow(deprecated)]
     target
         .fetch_update(Ordering::Relaxed, Ordering::Relaxed, |x| {
             if x == pred {

base-commit: 6c7577041b6a76ee4e18c3910bab12a268df037e
--
2.55.0