net: NULL the old xps map entries when freeing them

In __netif_set_xps_queue, old map entries from the old dev_maps are
freed but their corresponding entry in the old dev_maps aren't NULLed.
Fix this.
Signed-off-by: Antoine Tenart <atenart@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/dev.c

@@ -2764,6 +2764,7 @@ int __netif_set_xps_queue(struct net_device *dev, const unsigned long *mask,
 					continue;
 			}
 
+			RCU_INIT_POINTER(dev_maps->attr_map[tci], NULL);
 			kfree_rcu(map, rcu);
 		}
 	}