net: Fix unwanted sign extension in netdev_stats_to_stats64()

When converting net_device_stats to rtnl_link_stats64 sign extension
is triggered on ILP32 machines as 6c1c5097 changed the previous
"ulong -> u64" conversion to "long -> u64" by accessing the
net_device_stats fields through a (signed) atomic_long_t.

This causes for example the received bytes counter to jump to 16EiB after
having received 2^31 bytes. Casting the atomic value to "unsigned long"
beforehand converting it into u64 avoids this.

Fixes: 6c1c5097 ("net: add atomic_long_t to net_device_stats fields")
Signed-off-by: Felix Riemann <felix.riemann@sma.de>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/dev.c

@@ -10375,7 +10375,7 @@ void netdev_stats_to_stats64(struct rtnl_link_stats64 *stats64,
 
 	BUILD_BUG_ON(n > sizeof(*stats64) / sizeof(u64));
 	for (i = 0; i < n; i++)
-		dst[i] = atomic_long_read(&src[i]);
+		dst[i] = (unsigned long)atomic_long_read(&src[i]);
 	/* zero out counters that only exist in rtnl_link_stats64 */
 	memset((char *)stats64 + n * sizeof(u64), 0,
 	       sizeof(*stats64) - n * sizeof(u64));