vxlan: add ndo_change_proto_down support

Add ndo_change_proto_down support through dev_change_proto_down_generic
for use by control protocols like VRRPD.

Signed-off-by: Andy Roulin <aroulin@cumulusnetworks.com>
Acked-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>