This patch adds support to set and display protodown on a switch port. The
switch driver can handle this error state by doing a phys down on the port.

One example user space application setting this flag is a multi-chassis
LAG application to handle split-brain situation on peer-link failure.

Example:
root@net-next:~# ip link set eth1 protodown on
root@net-next:~/iproute2# ip link show eth1
4: eth1: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN mode DEFAULT group default qlen 1000
    link/ether 52:54:00:12:35:01 brd ff:ff:ff:ff:ff:ff protodown on
root@net-next:~/iproute2# ip link set eth1 protodown off
root@net-next:~/iproute2# ip link show eth1
4: eth1: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN mode DEFAULT group default qlen 1000
    link/ether 52:54:00:12:35:01 brd ff:ff:ff:ff:ff:ff
root@net-next:~/iproute2#
Signed-off-by: Anuradha Karuppiah <anuradhak@cumulusnetworks.com>
Signed-off-by: Andy Gospodarek <gospo@cumulusnetworks.com>
Signed-off-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: Wilson Kok <wkok@cumulusnetworks.com>