btrfs-progs: device usage: implement json format output

[jelly]

Implements JSON formatted output for the device usage command.

---

Tested on a single nvme btrfs filesystem and raid 1:

[jelle@homeserver ~]$ sudo ./btrfs --format json device usage  /mnt/disk /
{
  "__header": {
    "version": "1"
  },
  "device-usage": [
    {
      "device": "/dev/sda",
      "devid": 1,
      "device_size": 20000588955648,
      "device_slack": 0,
      "chunks": [
        {
          "type": "Data",
          "profile": "RAID1",
          "num_stripes": 2,
          "size": 12590696628224
        },
        {
          "type": "Metadata",
          "profile": "RAID1",
          "num_stripes": 2,
          "size": 16106127360
        },
        {
          "type": "System",
          "profile": "RAID1",
          "num_stripes": 2,
          "size": 8388608
        }
      ],
      "unallocated": 7393777811456
    },
    {
      "device": "/dev/sdb",
      "devid": 2,
      "device_size": 20000588955648,
      "device_slack": 0,
      "chunks": [
        {
          "type": "Data",
          "profile": "RAID1",
          "num_stripes": 2,
          "size": 12590696628224
        },
        {
          "type": "Metadata",
          "profile": "RAID1",
          "num_stripes": 2,
          "size": 16106127360
        },
        {
          "type": "System",
          "profile": "RAID1",
          "num_stripes": 2,
          "size": 8388608
        }
      ],
      "unallocated": 7393777811456
    },
    {
      "device": "/dev/nvme0n1p2",
      "devid": 1,
      "device_size": 999627103744,
      "device_slack": 3584,
      "chunks": [
        {
          "type": "Data",
          "profile": "single",
          "num_stripes": 1,
          "size": 312467259392
        },
        {
          "type": "Metadata",
          "profile": "DUP",
          "num_stripes": 2,
          "size": 6442450944
        },
        {
          "type": "System",
          "profile": "DUP",
          "num_stripes": 2,
          "size": 134217728
        }
      ],
      "unallocated": 680583172096
    }
  ]
}

[jelly]

Note to self, should update Documentation/dev/dev-json.rst as well.

[jelly]

I have decided to not report 0, similar to how zone_unusable is handled.