l2tp: Handle eth stats using NETDEV_PCPU_STAT_DSTATS.

l2tp_eth uses the TSTATS infrastructure (dev_sw_netstats_*()) for RX
and TX packet counters and DEV_STATS_INC for dropped counters.

Consolidate that using the DSTATS infrastructure, which can
handle both packet counters and packet drops. Statistics that don't
fit DSTATS are still updated atomically with DEV_STATS_INC().

This change is inspired by the introduction of DSTATS helpers and
their use in other udp tunnel drivers:
Link: https://lore.kernel.org/all/cover.1733313925.git.gnault@redhat.com/



Signed-off-by: James Chapman <jchapman@katalix.com>
Reviewed-by: Guillaume Nault <gnault@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>