Claude review: rust: gpuvm: add GpuVm::obtain()

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

Patch Review

Introduces `GpuVmBo<T>`, `GpuVmBoAlloc<T>`, and the `obtain()` path.

**Key concern — resv lock in `obtain()`:**
```rust
if ptr::eq(ptr, me.as_raw()) && me.gpuvm().is_extobj(me.obj()) {
    let resv_lock = me.gpuvm().raw_resv();
    // TODO: Use a proper lock guard here once a dma_resv lock abstraction exists.
    unsafe { bindings::dma_resv_lock(resv_lock, ptr::null_mut()) };
    unsafe { bindings::drm_gpuvm_bo_extobj_add(ptr) };
    unsafe { bindings::dma_resv_unlock(resv_lock) };
}
```
Passing `null` for `ww_acquire_ctx` means this cannot participate in deadlock avoidance. If a driver calls `obtain()` while already holding another resv lock in a ww transaction, this will deadlock. The TODO acknowledges this but it should be documented more prominently as a limitation, possibly with a `// WARNING` or a doc comment on `obtain()` itself.

**`GpuVmBoAlloc::Drop` — could use `drm_gpuvm_bo_destroy_not_in_lists()`:**
```rust
fn drop(&mut self) {
    // TODO: Call drm_gpuvm_bo_destroy_not_in_lists() directly.
    unsafe { bindings::drm_gpuvm_bo_put_deferred(self.as_raw()) };
}
```
The TODO is noted. Using `drm_gpuvm_bo_put_deferred` here works but does unnecessary refcount work for an object known to have refcount=1 and not be in any lists.

**ALLOC_FN optimization is clever:**
```rust
pub(super) const ALLOC_FN: Option<unsafe extern "C" fn() -> *mut bindings::drm_gpuvm_bo> = {
    ...
    if base.size() != rust.size() || base.align() != rust.align() {
        Some(Self::vm_bo_alloc)
    } else {
        None
    }
};
```
This correctly avoids overriding when the Rust wrapper has no extra fields. However, the `FREE_FN` check uses `needs_drop::<Self>()` which is independent — when `VmBoData` has a trivial type but non-default layout, you'd get `ALLOC_FN = Some` but `FREE_FN = None`, meaning the C side would `kfree()` a larger allocation which is fine since it was `kmalloc`'d. This works correctly.

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Generated by Claude Code Patch Reviewer