zebra: Fix JSON output for `show route summary json`

zebra: Fix JSON output for `show route summary json`

Before:
```
{
  "routes":[
    {
      "fib":1,
      "rib":1,
      "fibOffLoaded":0,
      "fibTrapped":0,
      "type":"connected"
    },
    {
      "fib":1,
      "rib":1,
      "fibOffLoaded":0,
      "fibTrapped":0,
      "type":"local"
    },
    {
      "fib":2,
      "rib":2,
      "fibOffLoaded":0,
      "fibTrapped":0,
      "type":"static"
    }
  ],
  "routesTotal":4,
  "routesTotalFib":4
}
{
  "routes":[
    {
      "fib":2,
      "rib":2,
      "fibOffLoaded":0,
      "fibTrapped":0,
      "type":"connected"
    },
    {
      "fib":2,
      "rib":2,
      "fibOffLoaded":0,
      "fibTrapped":0,
      "type":"local"
    },
    {
      "fib":2,
      "rib":2,
      "fibOffLoaded":0,
      "fibTrapped":0,
      "type":"static"
    }
  ],
  "routesTotal":6,
  "routesTotalFib":6
}
```

After:
```
{
  "ISR":{
    "routes":[
      {
        "fib":1,
        "rib":1,
        "fibOffLoaded":0,
        "fibTrapped":0,
        "type":"connected"
      },
      {
        "fib":1,
        "rib":1,
        "fibOffLoaded":0,
        "fibTrapped":0,
        "type":"local"
      },
      {
        "fib":2,
        "rib":2,
        "fibOffLoaded":0,
        "fibTrapped":0,
        "type":"static"
      }
    ],
    "routesTotal":4,
    "routesTotalFib":4
  },
  "default":{
    "routes":[
      {
        "fib":2,
        "rib":2,
        "fibOffLoaded":0,
        "fibTrapped":0,
        "type":"connected"
      },
      {
        "fib":2,
        "rib":2,
        "fibOffLoaded":0,
        "fibTrapped":0,
        "type":"local"
      },
      {
        "fib":2,
        "rib":2,
        "fibOffLoaded":0,
        "fibTrapped":0,
        "type":"static"
      }
    ],
    "routesTotal":6,
    "routesTotalFib":6
  }
}
```

Signed-off-by: Donatas Abraitis <donatas@opensourcerouting.org>