net: add new netdevice feature for offload of RX port for UDP tunnels

This adds a new netdevice feature, so that the offloading of RX port for
UDP tunnels can be disabled by the administrator on some netdevices,
using the "rx-udp_tunnel-port-offload" feature in ethtool.

This feature is set for all devices that provide ndo_udp_tunnel_add.

Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: David S. Miller <davem@davemloft.net>