Claude review: rust: add warn_on_err macro

[Claude Code Review Bot <claude-review@example.com>]

Patch Review

**File:** `rust/kernel/bug.rs`

```rust
+/// Report a warning if `res` is an error and return it unmodified.
+#[macro_export]
+macro_rules! warn_on_err {
+    ($res:expr) => {{
+        let res = $res;
+        let _ = $crate::warn_on!(res.is_err());
+        res
+    }};
+}
```

Clean and straightforward. Evaluates `$res` exactly once, delegates to the existing `warn_on!` macro, and returns the result unmodified. The `let _ =` discard of `warn_on!`'s return value is correct since we only care about the side effect (the WARN).

One minor point: this macro lives in `kernel/bug.rs` which is a shared crate-level utility. Since it's `#[macro_export]`, it becomes part of the `kernel` crate's public API. The Rust-for-Linux maintainers may want to weigh in on whether this is general enough to warrant that placement vs. being a driver-local helper. But given that `warn_on!` is already there, adding `warn_on_err!` alongside it is natural.

No issues.

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Generated by Claude Code Patch Reviewer