iplink: add support for protodown reason

iplink: add support for protodown reason

This patch adds support for recently
added link IFLA_PROTO_DOWN_REASON attribute.
IFLA_PROTO_DOWN_REASON enumerates reasons
for the already existing IFLA_PROTO_DOWN link
attribute.

$ cat /etc/iproute2/protodown_reasons.d/r.conf
0 mlag
1 evpn
2 vrrp
3 psecurity

$ ip link set dev vx10 protodown on protodown_reason vrrp on
$ip link show dev vx10
14: vx10: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN mode
DEFAULT group default qlen 1000
    link/ether f2:32:28:b8:35:ff brd ff:ff:ff:ff:ff:ff protodown on
protodown_reason <vrrp>
$ip -p -j link show dev vx10
[ {
	<snip>
        "proto_down": true,
        "proto_down_reason": [ "vrrp" ]
} ]
$ip link set dev vx10 protodown_reason mlag on
$ip link show dev vx10
14: vx10: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN mode
DEFAULT group default qlen 1000
    link/ether f2:32:28:b8:35:ff brd ff:ff:ff:ff:ff:ff protodown on
protodown_reason <mlag,vrrp>
$ip -p -j link show dev vx10
[ {
	<snip>
        "proto_down": true,
        "protodown_reason": [ "mlag","vrrp" ]
} ]

$ip -p -j link show dev vx10
$ip link set dev vx10 protodown off protodown_reason vrrp off
Error: Cannot clear protodown, active reasons.
$ip link set dev vx10 protodown off protodown_reason mlag off
$

Note: for somereason the json and non-json key for protodown
are different (protodown and proto_down). I have kept the
same for protodown reason for consistency (protodown_reason and
proto_down_reason).

Signed-off-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: David Ahern <dsahern@gmail.com>