[IPV6] SIT: Sparse: Use NULL pointer instead of 0.

| net/ipv6/sit.c:382:42: warning: Using plain integer as NULL pointer
Signed-off-by: NYOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>

net/ipv6/sit.c

@@ -379,7 +379,7 @@ static void ipip6_tunnel_uninit(struct net_device *dev)
 		dev_put(dev);
 	} else {
 		ipip6_tunnel_unlink(netdev_priv(dev));
-		ipip6_tunnel_del_prl(netdev_priv(dev), 0);
+		ipip6_tunnel_del_prl(netdev_priv(dev), NULL);
 		dev_put(dev);
 	}
 }