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

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Patch Review

**Purpose:** Adds `dmem_cgroup_below_min()` and `dmem_cgroup_below_low()` to query whether a pool's usage is within its protection limits. These mirror memcg's `mem_cgroup_below_{min,low}`.

**Review:** Clean and straightforward. The functions follow the established memcg pattern. The key difference from memcg (calling `dmem_cgroup_calculate_protection()` explicitly) is well-documented.

Minor nit — the comment uses the word "pendant" (German for counterpart):
```c
 * In mem_cgroup_below_min(), the memcg pendant, this call is missing.
```
Should be "counterpart" in English. This appears in both functions.

The two functions are nearly identical except for `emin` vs `elow`:
```c
return page_counter_read(&test->cnt) <= READ_ONCE(test->cnt.emin);
```
vs:
```c
return page_counter_read(&test->cnt) <= READ_ONCE(test->cnt.elow);
```
This duplication follows the memcg convention so it's acceptable, but a shared helper with a flag parameter could reduce duplication.

The `#else` stubs returning `false` are correct — without cgroup support, nothing is "protected."

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