Claude review: drm/v3d: Hold v3d_stats references in each job

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

Patch Review

This is the key patch that eliminates the race. By having each job hold its own kref'd references to both `client_stats` and `global_stats`, the stats pointers in the job are guaranteed valid for the job's entire lifetime, regardless of fd closure.

The refactoring of `v3d_job_start_stats` and `v3d_job_update_stats` to no longer take a queue parameter is clean - the job already knows its stats pointers.

The removal of the `queue_lock` / `file_priv` NULL check dance in `v3d_job_update_stats` is the main payoff:

```c
-	/* Don't update the local stats if the file context has already closed */
-	spin_lock(&queue->queue_lock);
-	if (job->file_priv)
-		v3d_stats_update(job->file_priv->stats[q], now);
-	spin_unlock(&queue->queue_lock);
```

becomes simply:

```c
	v3d_stats_update(job->client_stats, now);
```

This is safe because the job's kref keeps the stats object alive.

The stats refs are taken in `v3d_job_init()` after `kref_init(&job->refcount)` and released in `v3d_job_free()`. The placement looks correct.

No issues.

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