Claude review: drm/ras: Introduce get-error-threshold

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

Patch Review

Adds the `GET_ERROR_THRESHOLD` netlink command to drm_ras. The implementation follows the existing `GET_ERROR_COUNTER` pattern closely.

**Issue: `get-error-threshold` requires `admin-perm` for a read-only query.**

```yaml
      flags: [admin-perm]
```

The existing `get-error-counter` is a dump operation without `admin-perm`. It's worth questioning whether a threshold query truly requires admin privileges. If the intent is to keep parity with `set-error-threshold` that's one thing, but for a read-only query it seems unnecessarily restrictive compared to the existing counter read path.

**Minor: No `dumpit` handler for thresholds.**

Unlike `get-error-counter` which has both `doit` and `dumpit`, `get-error-threshold` only has `doit`. If a user wants to query thresholds for all errors across all nodes, they need to iterate manually. This may be intentional but is worth noting for completeness.

The core implementation is solid — proper cleanup paths with `genlmsg_cancel`/`nlmsg_free` on errors, correct attribute checking, and the `EOPNOTSUPP` return when the callback is absent.

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