Claude review: rust: drm: gem: shmem: Add DmaResvGuard helper

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

Patch Review

**Verdict: Minor concerns.**

The RAII guard pattern is correct. However:

**Return value of `dma_resv_lock()` is discarded:**

```rust
fn new(obj: &'a Object<T, C>) -> Self {
    // SAFETY: This lock is initialized throughout the lifetime of `object`.
    unsafe { bindings::dma_resv_lock(obj.raw_dma_resv(), ptr::null_mut()) };
    Self(obj)
}
```

`dma_resv_lock()` returns `int` (it wraps `ww_mutex_lock()`). With `ctx = NULL`, it degrades to a plain mutex lock that always returns 0, so this is safe in practice -- and matches the C-side pattern in `drm_gem_shmem_release()` which also discards the return. But the safety comment should mention *why* the return value is ignored (i.e., NULL ctx guarantees success). Future readers won't necessarily know this.

**The `#[expect(unused)]` is removed in patch 3** -- fine, but introducing a dead-code function in one patch just to use it in the next is slightly awkward for bisection. The function has no callers between patches 2 and 3. Not a blocker.

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Generated by Claude Code Patch Reviewer