sched/core: Fix set_user_nice()

Almost all scheduler functions update state with the following
pattern:

	if (queued)
		dequeue_task(rq, p, DEQUEUE_SAVE);
	if (running)
		put_prev_task(rq, p);

	/* update state */

	if (queued)
		enqueue_task(rq, p, ENQUEUE_RESTORE);
	if (running)
		set_curr_task(rq, p);

set_user_nice() however misses the running part, cure this.

This was found by asserting we never enqueue 'current'.
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/sched/core.c

@@ -3724,7 +3724,8 @@ void rt_mutex_setprio(struct task_struct *p, int prio)
 
 void set_user_nice(struct task_struct *p, long nice)
 {
-	int old_prio, delta, queued;
+	bool queued, running;
+	int old_prio, delta;
 	struct rq_flags rf;
 	struct rq *rq;
 
@@ -3746,8 +3747,11 @@ void set_user_nice(struct task_struct *p, long nice)
 		goto out_unlock;
 	}
 	queued = task_on_rq_queued(p);
+	running = task_current(rq, p);
 	if (queued)
 		dequeue_task(rq, p, DEQUEUE_SAVE);
+	if (running)
+		put_prev_task(rq, p);
 
 	p->static_prio = NICE_TO_PRIO(nice);
 	set_load_weight(p);
@@ -3764,6 +3768,8 @@ void set_user_nice(struct task_struct *p, long nice)
 		if (delta < 0 || (delta > 0 && task_running(rq, p)))
 			resched_curr(rq);
 	}
+	if (running)
+		set_curr_task(rq, p);
 out_unlock:
 	task_rq_unlock(rq, p, &rf);
 }