iproute_lwtunnel: fix JSON output

iproute_lwtunnel: fix JSON output

The same tag "dst" was being used for both the route destination
and the encap destination. This made it hard for JSON parsers.
Change to put the per-encap information under a nested JSON
object (similar to ip link type info).

Original output
[ {
        "dst": "192.168.11.0/24",
        "encap": "ip6",
        "id": 0,
        "src": "::",
        "dst": "fd00::c0a8:2dd",
        "hoplimit": 0,
        "tc": 0,
        "protocol": "5",
        "scope": "link",
        "flags": [ ]
    } ]

Revised output
[ {
        "dst": "192.168.11.0/24",
        "encap": {
            "encap_type": "ip6",
            "id": 0,
            "src": "::",
            "dst": "fd00::c0a8:2dd",
            "hoplimit": 0,
            "tc": 0
        },
        "protocol": "5",
        "scope": "link",
        "flags": [ ]
    } ]

Reported-by: Lars Ekman <uablrek@gmail.com>
Fixes: 663c3cb23103 ("iproute: implement JSON and color output")
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>