Claude review: rust: gpuvm: add GpuVa struct

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

Patch Review

Introduces `GpuVa<T>`, `GpuVaAlloc<T>`, and `GpuVaRemoved<T>` -- the lifecycle types for GPU virtual address mappings.

**Observations:**

1. **`GpuVaAlloc::prepare` ownership transfer**: This method consumes `self` (a `KBox<MaybeUninit<GpuVa<T>>>`) and returns a raw pointer:
   ```rust
   pub(super) fn prepare(mut self, va_data: impl PinInit<T::VaData>) -> *mut bindings::drm_gpuva {
       let va_ptr = MaybeUninit::as_mut_ptr(&mut self.0);
       let Ok(()) = unsafe { va_data.__pinned_init(&raw mut (*va_ptr).data) };
       KBox::into_raw(self.0).cast()
   }
   ```
   After `KBox::into_raw`, the memory is no longer managed by Rust. The caller (`OpMap::insert` or `OpRemap::remap`) passes this pointer to C functions (`drm_gpuva_map`, `drm_gpuva_remap`) which take ownership. The pointer is eventually reclaimed via `GpuVaRemoved::from_raw` after `drm_gpuva_unmap`. This is correct but requires careful attention from the caller -- the `#[must_use]` annotation on `prepare` helps.

2. **`GpuVaRemoved::into_inner` requires `Unpin`**: This is necessary because `GpuVa<T>` has `#[pin] data: T::VaData` -- moving the `VaData` out of the pinned location requires `Unpin`. Correct.

3. **`GpuVaRemoved` derefs to `T::VaData`, not `GpuVa<T>`**: This is a design choice -- once removed, the VA's address/range are no longer meaningful (the mapping is gone), so only the driver data is exposed. This is a reasonable API choice.

4. **Missing `Drop` for `GpuVaAlloc`**: If a `GpuVaAlloc` is dropped without being consumed by `prepare()`, the `KBox<MaybeUninit<GpuVa<T>>>` is dropped. Since the `inner` (the `drm_gpuva`) was zeroed but never initialized by `drm_gpuva_init` or similar, and the `data` field was never initialized either, dropping `MaybeUninit` is correct -- it doesn't run drop on the contents. This is fine.

5. **`VaData` added to `DriverGpuVm` trait**: The associated type `VaData` has no bounds. This is consistent with `VmBoData` which also has no bounds. Drivers can add whatever data they need.

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Generated by Claude Code Patch Reviewer