sfc: configure UDP tunnel offload ports

Implement ndo_udp_tunnel_{add,del} to update the NIC's list of VXLAN and
 GENEVE UDP ports.  Also reset the port list to empty on driver load and
 on driver unload, with appropriate flag set on the unload case.
These port numbers are used for RX inner checksum offload, and in future
 will also be used for TX inner checksum offload and encapsulated TSO.

Signed-off-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>