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

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

Patch Review

**Status: Minor concerns**

The guard pattern is correct in structure:

```rust
+struct DmaResvGuard<'a, T: DriverObject>(&'a Object<T>);
+
+impl<'a, T: DriverObject> DmaResvGuard<'a, T> {
+    fn new(obj: &'a Object<T>) -> Self {
+        unsafe { bindings::dma_resv_lock(obj.raw_dma_resv(), ptr::null_mut()) };
+        Self(obj)
+    }
+}
```

**Concern 1**: `dma_resv_lock()` returns `int`, but the return value is silently discarded. With `ctx = NULL`, `ww_mutex_lock` reduces to a simple mutex lock that in practice always returns 0, but ignoring the return value is not ideal. Since this is called from `Drop` contexts (via `SGTableMap::drop` in patch 6) where returning errors isn't possible, at minimum a `WARN_ON` or `BUG_ON` on failure would be appropriate — or a comment explaining why the return is ignored.

**Concern 2**: The `#[expect(unused)]` attribute is removed in patch 6, which is correct for the series as a whole, but it means patch 5 in isolation adds dead code. This is a minor bisectability concern — consider squashing patches 5 and 6, or keeping `#[expect(unused)]` in patch 5 and removing it in patch 6.

The TODO comment about replacing with WwMutex is appropriate.

---
Generated by Claude Code Patch Reviewer