Claude review: drm/gem/shmem: Introduce __drm_gem_shmem_free_sgt_locked()

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

Patch Review

**Concern: Missing NULL check on `shmem->sgt`.**

The extracted function unconditionally dereferences `shmem->sgt`:

```c
void __drm_gem_shmem_free_sgt_locked(struct drm_gem_shmem_object *shmem)
{
	dma_resv_assert_held(shmem->base.resv);

	dma_unmap_sgtable(shmem->base.dev->dev, shmem->sgt, DMA_BIDIRECTIONAL, 0);
	sg_free_table(shmem->sgt);
	kfree(shmem->sgt);
	shmem->sgt = NULL;
}
```

The original code in `drm_gem_shmem_release()` wrapped this with `if (shmem->sgt)`. The call site in `drm_gem_shmem_release` still checks `if (shmem->sgt)` before calling the new function, and the Rust callers (patch 4) also only call it when the sgt is known-valid. So this is safe in practice, but since the function is exported and documented, it would be more defensive to either:
- Add a NULL check inside the function, or
- Document in the kdoc that `shmem->sgt` must be non-NULL.

Currently the kdoc says "If the passed shmem object has an active scatter/gather table..." which implies it handles the NULL case, but it doesn't.

**Minor: Doc/function name mismatch in commit message.** The commit message references `__drm_gem_shmem_release_sgt_locked()` in the V10 changelog, but the actual function is `__drm_gem_shmem_free_sgt_locked()`. This is just the changelog, not the code, so it's a cosmetic issue only.

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