net: l2tp_eth: use generic dev->stats fields

Core networking has opt-in atomic variant of dev->stats,
simply use DEV_STATS_INC(), DEV_STATS_ADD() and DEV_STATS_READ().

v2: removed @priv local var in l2tp_eth_dev_recv() (Simon)

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>