openvswitch: fix hangup on vxlan/gre/geneve device deletion

Each openvswitch tunnel vport (vxlan,gre,geneve) holds a reference
to the underlying tunnel device, but never released it when such
device is deleted.
Deleting the underlying device via the ip tool cause the kernel to
hangup in the netdev_wait_allrefs() loop.
This commit ensure that on device unregistration dp_detach_port_notify()
is called for all vports that hold the device reference, properly
releasing it.

Fixes: 614732ea ("openvswitch: Use regular VXLAN net_device device")
Fixes: b2acd1dc ("openvswitch: Use regular GRE net_device instead of vport")
Fixes: 6b001e68

 ("openvswitch: Use Geneve device.")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Acked-by: Flavio Leitner <fbl@sysclose.org>
Acked-by: Pravin B Shelar <pshelar@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>