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>