rcu: Stop shrinker loop

The count and scan can be separated in time, and there is a fair chance
that all work is already done when the scan starts, which might in turn
result in a needless retry.  This commit therefore avoids this retry by
returning SHRINK_STOP.
Reviewed-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Signed-off-by: Peter Enderborg <peter.enderborg@sony.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/rcu/tree.c

@@ -3332,7 +3332,7 @@ kfree_rcu_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
 			break;
 	}
 
-	return freed;
+	return freed == 0 ? SHRINK_STOP : freed;
 }
 
 static struct shrinker kfree_rcu_shrinker = {