net: add infrastructure to un-offload UDP tunnel port

This adds a new NETDEV_UDP_TUNNEL_DROP_INFO event, similar to
NETDEV_UDP_TUNNEL_PUSH_INFO, to signal to un-offload ports.

This also adds udp_tunnel_drop_rx_port(), which calls
ndo_udp_tunnel_del.

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