net: sched: act_tunnel_key: request UDP checksum by default

There's currently no way to request (outer) UDP checksum with
act_tunnel_key. This is problem especially for IPv6. Right now, tunnel_key
action with IPv6 does not work without going through hassles: both sides
have to have udp6zerocsumrx configured on the tunnel interface. This is
obviously not a good solution universally.

It makes more sense to compute the UDP checksum by default even for IPv4.
Just set the default to request the checksum when using act_tunnel_key.

Signed-off-by: Jiri Benc <jbenc@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>