l2tp: l2tp_eth: use per-cpu counters from dev->tstats

l2tp_eth uses old-style dev->stats for fastpath packet/byte
counters. Convert it to use dev->tstats per-cpu counters.

Signed-off-by: James Chapman <jchapman@katalix.com>
Signed-off-by: Tom Parkin <tparkin@katalix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>