ip: iplink_vlan.c: add json output support

ip: iplink_vlan.c: add json output support

Schema:
{
    "protocol": {
        "type": "string",
        "attr": "IFLA_VLAN_PROTOCOL"
    },
    "id": {
        "type": "uint",
        "attr": "IFLA_VLAN_ID"
    },
    "flags": {
        "type": "array",
        "attr": "IFLA_VLAN_FLAGS",
        "array": [
            {
                "type": "string"
            }
        ]
    },
    "ingress_qos": {
        "type": "array",
        "attr": "IFLA_VLAN_INGRESS_QOS",
        "array": [
            {
                "type": "dict",
                "dict": {
                    "from": {
                        "type": "uint"
                    },
                    "to": {
                        "type": "uint"
                    }
                }
            }
        ]
    },
    "egress_qos": {
        "type": "array",
        "attr": "IFLA_VLAN_EGRESS_QOS",
        "array": [
            {
                "type": "dict",
                "dict": {
                    "from": {
                        "type": "uint"
                    },
                    "to": {
                        "type": "uint"
                    }
                }
            }
        ]
    }
}

$ ip link add name eth0.42 link eth0 type vlan id 42
$ ip -details -json link show
[{
        "ifindex": 30,
        "ifname": "eth0.42",
        "link": "eth0",
        "flags": ["BROADCAST","MULTICAST"],
        "mtu": 1500,
        "qdisc": "noop",
        "operstate": "DOWN",
        "linkmode": "DEFAULT",
        "group": "default",
        "link_type": "ether",
        "address": "08:00:27:db:31:88",
        "broadcast": "ff:ff:ff:ff:ff:ff",
        "promiscuity": 0,
        "linkinfo": {
            "info_kind": "vlan",
            "info_data": {
                "protocol": "802.1Q",
                "id": 42,
                "flags": ["REORDER_HDR"]
            }
        },
        "inet6_addr_gen_mode": "eui64",
        "num_tx_queues": 1,
        "num_rx_queues": 1,
        "gso_max_size": 65536,
        "gso_max_segs": 65535
    }
]

Signed-off-by: Julien Fortin <julien@cumulusnetworks.com>