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.