geneve: add dst caching support

use generic dst implementation for both plain geneve devices and
lwtunnels.

In case of UDP traffic with datagram length below MTU this give
about 2% performance increase for plain geneve tunnel over ipv4,
about 65% performance increase for ipv6 tunnel.

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Suggested-and-Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>