geneve/vxlan: offload ports on register/unregister events

This improves consistency of handling when moving a netdev to another
netns. Most drivers currently do a full reset when the device goes up,
so that will flush the offload state anyway.

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