Claude review: drm/vmwgfx: Reserve ttm object before resv usage

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

Patch Review

**Purpose:** Fix `dma_resv_assert_held` warnings by reserving the TTM buffer object before operations that require the reservation to be held.

**Issues:**

1. **`ttm_bo_reserve` in `vmw_bo_free` — potential deadlock concern:** In `vmw_bo_free`, the buffer object is being freed (refcount is 0, per the `WARN_ON` at line 45). Calling `ttm_bo_reserve(bo, false, false, NULL)` on a BO whose refcount is already 0 is unusual. The reservation is typically held by something with a live reference. Since this is a teardown path and no other thread should hold a reference, the reservation should succeed immediately, but this pattern is fragile. If anything else tries to acquire the reservation concurrently (which shouldn't happen given refcount==0), it could deadlock with `interruptible=false, no_wait=false`.

```c
ttm_bo_reserve(bo, false, false, NULL);
vmw_resource_mob_detach(res);
ttm_bo_unreserve(bo);
```

Consider whether `dma_resv_lock` directly (without going through `ttm_bo_reserve`) would be more appropriate here, or whether the callers already guarantee the reservation is held and the real fix should be elsewhere.

2. **Missing Fixes tag:** No `Fixes:` tag on this patch, despite addressing concrete warnings.

3. **VKMS CRC worker — `goto done` skips `compute_crc` but still unreferences:** The error handling in `crc_generate_worker` looks correct — on reserve failure, it jumps to `done:` which calls `vmw_surface_unreference`, which is the right cleanup. However, the blank line after `done:` is unnecessary:

```c
done:

	vmw_surface_unreference(&surf);
```

4. **VKMS reserve — is `guest_memory_bo` guaranteed non-NULL?** The code does:
```c
ret = ttm_bo_reserve(&surf->res.guest_memory_bo->tbo, false, false, NULL);
```
If `surf->res.guest_memory_bo` could be NULL, this would oops. Looking at the flow, `vmw_surface_sync` succeeded so the surface should have a valid backing BO, but a NULL check or assertion might be prudent.

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