ip: iplink_geneve.c: add json output support

ip: iplink_geneve.c: add json output support

Schema:
{
    "id": {
        "type": "uint",
        "attr": "IFLA_GENEVE_ID"
    },
    "remote": {
        "type": "string",
        "attr": "IFLA_GENEVE_REMOTE"
    },
    "remote6": {
        "type": "string",
        "attr": "IFLA_GENEVE_REMOTE6"
    },
    "ttl": {
        "type": "int",
        "attr": "IFLA_GENEVE_TTL"
    },
    "tos": {
        "type": "string",
        "attr": "IFLA_GENEVE_TOS"
    },
    "label": {
        "type": "string",
        "attr": "IFLA_GENEVE_LABEL"
    },
    "port": {
        "type": "uint",
        "attr": "IFLA_GENEVE_PORT"
    },
    "collect_metadata": {
        "type": "bool",
        "attr": "IFLA_GENEVE_COLLECT_METADATA"
    },
    "udp_csum": {
        "type": "bool",
        "attr": "IFLA_GENEVE_UDP_CSUM"
    },
    "udp_zero_csum6_tx": {
        "type": "bool",
        "attr": "IFLA_GENEVE_UDP_ZERO_CSUM6_TX"
    },
    "udp_zero_csum6_rx": {
        "type": "bool",
        "attr": "IFLA_GENEVE_UDP_ZERO_CSUM6_RX"
    }
}

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