workqueue: Rename rescuer kworker

Each CPU-specific and unbound kworker kthread conforms to a particular
naming scheme. However, this does not extend to the rescuer kworker.
At present, a rescuer kworker is simply named according to its
workqueue's name. This can be cryptic.

This patch modifies a rescuer to follow the kworker naming scheme.
The "R" is indicative of a rescuer and after "-" is its workqueue's
name e.g. "kworker/R-ext4-rsv-conver".

tj: Use "R" instead of "r" as the prefix to make it more distinctive and
    consistent with how highpri pools are marked.
Signed-off-by: Aaron Tomlin <atomlin@atomlin.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/workqueue.c

@@ -4642,7 +4642,7 @@ static int init_rescuer(struct workqueue_struct *wq)
 	}
 
 	rescuer->rescue_wq = wq;
-	rescuer->task = kthread_create(rescuer_thread, rescuer, "%s", wq->name);
+	rescuer->task = kthread_create(rescuer_thread, rescuer, "kworker/R-%s", wq->name);
 	if (IS_ERR(rescuer->task)) {
 		ret = PTR_ERR(rescuer->task);
 		pr_err("workqueue: Failed to create a rescuer kthread for wq \"%s\": %pe",