ip6_tunnel: use ip_tunnel_get_stats64 as ndo_get_stats64 callback

Switch ip6_tunnel to the standard statistics pattern:
- use dev->stats for the less frequently accessed counters
- use dev->tstats for the frequently accessed counters

An additional benefit is that we now have 64bit statistics also on
32bit systems.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>