ipv6: tunnels: use DEV_STATS_INC()

Most of code paths in tunnels are lockless (eg NETIF_F_LLTX in tx).

Adopt SMP safe DEV_STATS_{INC|ADD}() to update dev->stats fields.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>