net: accept an empty mask in /sys/class/net/*/queues/rx-*/rps_cpus

We must accept an empty mask in store_rps_map(), or we are not able
to disable RPS on a queue.

Fixes: 07bbecb ("net: Restrict receive packets queuing to housekeeping CPUs")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Maciej Żenczykowski <maze@google.com>
Cc: Alex Belits <abelits@marvell.com>
Cc: Nitesh Narayan Lal <nitesh@redhat.com>
Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Maciej Żenczykowski <maze@google.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Nitesh Narayan Lal <nitesh@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/net-sysfs.c

@@ -757,11 +757,13 @@ static ssize_t store_rps_map(struct netdev_rx_queue *queue,
 		return err;
 	}
 
-	hk_flags = HK_FLAG_DOMAIN | HK_FLAG_WQ;
-	cpumask_and(mask, mask, housekeeping_cpumask(hk_flags));
-	if (cpumask_empty(mask)) {
-		free_cpumask_var(mask);
-		return -EINVAL;
+	if (!cpumask_empty(mask)) {
+		hk_flags = HK_FLAG_DOMAIN | HK_FLAG_WQ;
+		cpumask_and(mask, mask, housekeeping_cpumask(hk_flags));
+		if (cpumask_empty(mask)) {
+			free_cpumask_var(mask);
+			return -EINVAL;
+		}
 	}
 
 	map = kzalloc(max_t(unsigned int,