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

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

Patch Review

**Bug — return value of `dma_resv_lock` is ignored:**

```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`. With `ctx` as NULL, `ww_mutex_lock` should succeed (no deadlock detection context), but it can still return `-EALREADY` if the caller already holds the lock. Ignoring the return value means on failure, the `Drop` impl will call `dma_resv_unlock()` on a lock that was never acquired — undefined behavior. At minimum, `new()` should return `Result<Self>` and propagate the error.

**Minor — `#[expect(unused)]` on `new()`:** This attribute is removed in patch 2, which is fine, but the helper itself is dead code in this patch. Consider squashing patches 1 and 2 since the guard has no users until patch 2.

**Comment nit:** The TODO comment about WwMutex is good forward-looking documentation.

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