Claude review: rust: gpuvm: add GpuVmCore::sm_map()

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

Patch Review

Adds the map operation, completing the split/merge functionality.

**`OpMap::insert` drops the gpuva guard immediately:**
```rust
pub fn insert(self, va: GpuVaAlloc<T>, va_data: impl PinInit<T::VaData>) -> OpMapped<'op, T> {
    let va = va.prepare(va_data);
    unsafe { bindings::drm_gpuva_map(self.vm_bo.gpuvm().as_raw(), va, self.op) };
    let _gpuva_guard = self.vm_bo().lock_gpuva();
    unsafe { bindings::drm_gpuva_link(va, self.vm_bo.as_raw()) };
    OpMapped { _invariant: self._invariant }
}
```
The guard is correctly scoped — it's taken just for the `drm_gpuva_link` call and dropped when the function returns. The `_gpuva_guard` naming with underscore prefix would normally indicate an unused variable, but the `let` binding keeps it alive until end of scope. This is correct but could be clearer with a named binding like `let gpuva_guard = ...` (matching the style in `remap()` from patch 5 which uses `let gpuva_guard =`). Actually looking again, patch 5's `remap()` does use `gpuva_guard` without underscore. Minor inconsistency.

**The `SmMapData` → `SmData` cast is sound** due to `#[repr(C)]` on `SmMapData` ensuring `sm_data` is at offset 0. The `sm_step_unmap` and `sm_step_remap` callbacks cast the pointer to `SmData`, which works whether the actual data is `SmData` (from `sm_unmap`) or `SmMapData` (from `sm_map`). This is well-designed.

**`OpMapRequest` has public fields** — this is a clean API choice for constructing the request.

**Overall the series is in good shape** for a v5. The main actionable items are:
1. Fix the SAFETY comment typo in `GpuVa::obj()` ("gem.offset" → "gem.obj")
2. Consider adding `#[repr(C)]` to `SmData` for robustness
3. Document the `null` ww_acquire_ctx limitation on `obtain()` more prominently
4. Consider requiring `T: Send` for `GpuVm<T>: Send`

---
Generated by Claude Code Patch Reviewer