ip: link_vti6.c: add json output support

ip: link_vti6.c: add json output support

Schema:
{
    "remote": {
        "type": "string",
        "attr": "IFLA_VTI_REMOTE"
    },
    "local": {
        "type": "string",
        "attr": "IFLA_VTI_LOCAL"
    },
    "link": {
        "type": "string",
        "attr": "IFLA_VTI_LINK",
        "mutually_exclusive": {
            "link_index": {
                "type": "uint",
            }
        }
    },
    "ikey": {
        "type": "string",
        "attr": "IFLA_VTI_IKEY"
    },
    "okey": {
        "type": "string",
        "attr": "IFLA_VTI_OKEY"
    }
}

➜  ~ ip -6 tunnel add name vti6 mode vti6 local 2001:db8:1::1/64 remote
2001:0db8:85a3:0000:0000:8a2e:0370:7334
➜  ~ ip link show
10: ip6tnl0@NONE: <NOARP> mtu 1452 qdisc noop state DOWN mode DEFAULT
group default
    link/tunnel6 :: brd ::
11: ip6_vti0@NONE: <NOARP> mtu 1500 qdisc noop state DOWN mode DEFAULT
group default
    link/tunnel6 :: brd ::
12: vti6@NONE: <POINTOPOINT,NOARP> mtu 1500 qdisc noop state DOWN mode
DEFAULT group default
    link/tunnel6 2001:db8:1::1 peer 2001:db8:85a3::8a2e:370:7334
➜  ~ ./ip -details -json link show
[{
        "ifindex": 10,
        "ifname": "ip6tnl0",
        "link": null,
        "flags": ["NOARP"],
        "mtu": 1452,
        "qdisc": "noop",
        "operstate": "DOWN",
        "linkmode": "DEFAULT",
        "group": "default",
        "link_type": "tunnel6",
        "address": "::",
        "broadcast": "::",
        "promiscuity": 0,
        "linkinfo": {
            "info_kind": "ip6tnl",
            "info_data": {
                "proto": "ip6ip6",
                "remote": "::",
                "local": "::",
                "encap_limit": 0,
                "ttl": 0,
                "flowinfo_tclass": "0x00",
                "flowlabel": "0x00000",
                "flowinfo": "0x00000000"
            }
        },
        "inet6_addr_gen_mode": "eui64",
        "num_tx_queues": 1,
        "num_rx_queues": 1,
        "gso_max_size": 65536,
        "gso_max_segs": 65535
    },{
        "ifindex": 11,
        "ifname": "ip6_vti0",
        "link": null,
        "flags": ["NOARP"],
        "mtu": 1500,
        "qdisc": "noop",
        "operstate": "DOWN",
        "linkmode": "DEFAULT",
        "group": "default",
        "link_type": "tunnel6",
        "address": "::",
        "broadcast": "::",
        "promiscuity": 0,
        "linkinfo": {
            "info_kind": "vti6",
            "info_data": {
                "remote": "::",
                "local": "::",
                "ikey": "0.0.0.0",
                "okey": "0.0.0.0"
            }
        },
        "inet6_addr_gen_mode": "eui64",
        "num_tx_queues": 1,
        "num_rx_queues": 1,
        "gso_max_size": 65536,
        "gso_max_segs": 65535
    },{
        "ifindex": 12,
        "ifname": "vti6",
        "link": null,
        "flags": ["POINTOPOINT","NOARP"],
        "mtu": 1500,
        "qdisc": "noop",
        "operstate": "DOWN",
        "linkmode": "DEFAULT",
        "group": "default",
        "link_type": "tunnel6",
        "address": "2001:db8:1::1",
        "link_pointtopoint": true,
        "broadcast": "2001:db8:85a3::8a2e:370:7334",
        "promiscuity": 0,
        "linkinfo": {
            "info_kind": "vti6",
            "info_data": {
                "remote": "2001:db8:85a3::8a2e:370:7334",
                "local": "2001:db8:1::1",
                "ikey": "0.0.0.0",
                "okey": "0.0.0.0"
            }
        },
        "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>