net: dev: add generic protodown handler

Introduce dev_change_proto_down_generic, a generic ndo_change_proto_down
implementation, which sets the netdev carrier state according to proto_down.

This adds the ability to set protodown on vxlan and macvlan devices in a
generic way 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>