Claude review: drm/ttm: Split cgroup charge and resource allocation

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

Patch Review

This is an important correctness fix. The race is well-described: when charge and allocation are coupled, concurrent allocations from the same protected cgroup can all pass the min/low check because the charge is released when allocation fails, making the protection check unreliable.

The new `ttm_resource_try_charge()` is clean:

```c
+int ttm_resource_try_charge(struct ttm_buffer_object *bo,
+			    const struct ttm_place *place,
+			    struct dmem_cgroup_pool_state **ret_pool,
+			    struct dmem_cgroup_pool_state **ret_limit_pool)
```

The ownership transfer comment is good practice:

```c
+	/*
+	 * Ownership of charge_pool has been transferred to the TTM resource,
+	 * don't make the caller think we still hold a reference to it.
+	 */
+	alloc_state->charge_pool = NULL;
```

The caller correctly puts `charge_pool` on all error paths (including the `-EBUSY` path after `ttm_bo_evict_alloc` fails).

In `ttm_bo_evict_cb`, the allocation during eviction now correctly passes the held `charge_pool`:

```c
-					  evict_walk->res, NULL);
+					  evict_walk->res, evict_walk->alloc_state->charge_pool);
```

**Minor commit message typo**: "at a poin where" → "at a point where".

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