net/ipv4: add dst cache support for gre lwtunnels

In case of UDP traffic with datagram length below MTU this
gives about 4% performance increase

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>