Claude review: cgroup/dmem: Add queries for protection values

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

Patch Review

Clean addition of `dmem_cgroup_below_min()` and `dmem_cgroup_below_low()` as counterparts to memcg's equivalents.

The comment explaining why `dmem_cgroup_calculate_protection()` is needed here (unlike memcg which calculates during tree traversal) is excellent and very helpful for future maintainers:

```c
+	/*
+	 * In mem_cgroup_below_min(), the memcg pendant, this call is missing.
+	 * mem_cgroup_below_min() gets called during traversal of the cgroup tree, where
+	 * protection is already calculated as part of the traversal. dmem cgroup eviction
+	 * does not traverse the cgroup tree, so we need to recalculate effective protection
+	 * here.
+	 */
+	dmem_cgroup_calculate_protection(root, test);
```

The root-finding loop is correct and the early return for `root == test || !pool_parent(test)` is appropriate. The inline stubs returning `false` when `CONFIG_CGROUP_DMEM` is disabled are correct.

No issues with this patch.

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