ipv6/sit: use DEV_STATS_INC() to avoid data-races

In the Linux kernel, the following vulnerability has been resolved: ipv6/sit: use DEV_STATS_INC() to avoid data-races syzbot/KCSAN reported that multiple cpus are updating dev->stats.tx_error concurrently. This is because sit tunnels are NETIF_F_LLTX, meaning their ndo_start_xmit() is not protected by a spinlock. While original KCSAN report was about tx path, rx path has the same issue.