Claude review: drm/ttm: Be more aggressive when allocating below protection limit

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

Patch Review

This is the core behavioral change. The extensive comment block explaining the design rationale is well-written and thorough.

The protection logic is correct:

```c
+	may_evict |= dmem_cgroup_below_min(NULL, alloc_state->charge_pool);
+	below_low = dmem_cgroup_below_low(NULL, alloc_state->charge_pool);
+	alloc_state->only_evict_unprotected = !may_evict && below_low;
```

This correctly implements the priority hierarchy:
- **below_min**: aggressive eviction (even low-protected buffers) via `may_evict = true`
- **below_low only**: restricted eviction (only unprotected buffers) via `only_evict_unprotected = true`
- **force_space already set**: normal full eviction path (`only_evict_unprotected` stays false because `!may_evict` is false)

The `only_evict_unprotected` flag correctly prevents the retry-with-low logic in `ttm_bo_evict_alloc`:

```c
+	if (!lret && evict_walk.hit_low && !state->only_evict_unprotected) {
```

**Nit**: The multi-line comment style doesn't quite follow kernel convention:

```c
+	/* If we failed to find enough BOs to evict, but we skipped over
+	 * some BOs because they were covered by dmem low protection, retry
```

Kernel convention would be:

```c
+	/*
+	 * If we failed to find enough BOs to evict, but we skipped over
+	 * some BOs because they were covered by dmem low protection, retry
```

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