Claude review: drm/ttm: Use common ancestor of evictor and evictee as limit pool

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

Patch Review

**Purpose:** Uses the common ancestor of evictor and evictee as the root for protection calculation, enabling correct behavior for sibling cgroup prioritization.

**Self-eviction guard:**
```c
	if (evict_walk->alloc_state->only_evict_unprotected &&
			bo->resource->css == evict_walk->alloc_state->charge_pool)
		return 0;
```
This prevents a protected cgroup from evicting its own buffers during the "only unprotected" pass. The pointer comparison checks for exact same pool — not parent/child relationships. This is intentional: a cgroup should be allowed to evict from its own children, but not from itself when it's trying to enforce protection.

**Common ancestor logic:** Well-motivated by the comment:
```c
	/*
	 * ...Recursive protection distributes cgroup protection afforded
	 * to a parent cgroup but not used explicitly by a child cgroup between
	 * all child cgroups...
	 */
```
Without this, two sibling cgroups where one is protected and the other isn't would both appear equally protected (due to recursive distribution from their common parent), preventing the protected one from stealing from the unprotected one.

**Reference management:**
```c
	ancestor = dmem_cgroup_common_ancestor(bo->resource->css,
					       evict_walk->alloc_state->charge_pool);
	limit_pool = ancestor;
	...
	evict_valuable = dmem_cgroup_state_evict_valuable(limit_pool, ...);
	if (ancestor)
		dmem_cgroup_pool_state_put(ancestor);
```
The `put` after use is correct, but relies on `dmem_cgroup_common_ancestor()` returning a referenced pool (see concern in patch 2 review).

**Formatting nit:** The self-eviction check uses non-standard indentation:
```c
	if (evict_walk->alloc_state->only_evict_unprotected &&
			bo->resource->css == evict_walk->alloc_state->charge_pool)
```
Kernel style typically aligns continuation lines to the opening parenthesis, or uses a single tab indent. The double-tab here is inconsistent with the rest of the series.

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