l2tp: cast l2tp traffic counter to unsigned

commit 9b3dc0a1 upstream.

This fixes a counter problem on 32bit systems:
When the rx_bytes counter reached 2 GiB, it jumpd to (2^64 Bytes - 2GiB) Bytes.

rtnl_link_stats64 has __u64 type and atomic_long_read returns
atomic_long_t which is signed. Due to the conversation
we get an incorrect value on 32bit systems if the MSB of
the atomic_long_t value is set.

CC: Tom Parkin <tparkin@katalix.com>
Fixes: 7b7c0719 ("l2tp: avoid deadlock in l2tp stats update")
Signed-off-by: Dominik Heidler <dheidler@suse.de>
Signed-off-by: David S. Miller <davem@davemloft.net>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

net/l2tp/l2tp_eth.c

@@ -111,12 +111,13 @@ static struct rtnl_link_stats64 *l2tp_eth_get_stats64(struct net_device *dev,
 {
 	struct l2tp_eth *priv = netdev_priv(dev);
 
-	stats->tx_bytes   = atomic_long_read(&priv->tx_bytes);
-	stats->tx_packets = atomic_long_read(&priv->tx_packets);
-	stats->tx_dropped = atomic_long_read(&priv->tx_dropped);
-	stats->rx_bytes   = atomic_long_read(&priv->rx_bytes);
-	stats->rx_packets = atomic_long_read(&priv->rx_packets);
-	stats->rx_errors  = atomic_long_read(&priv->rx_errors);
+	stats->tx_bytes   = (unsigned long) atomic_long_read(&priv->tx_bytes);
+	stats->tx_packets = (unsigned long) atomic_long_read(&priv->tx_packets);
+	stats->tx_dropped = (unsigned long) atomic_long_read(&priv->tx_dropped);
+	stats->rx_bytes   = (unsigned long) atomic_long_read(&priv->rx_bytes);
+	stats->rx_packets = (unsigned long) atomic_long_read(&priv->rx_packets);
+	stats->rx_errors  = (unsigned long) atomic_long_read(&priv->rx_errors);
+
 	return stats;
 }