feat(list_snap): LIST_SNAP uzfs ioctl implementation

[vishnuitta]

This PR adds the LIST_SNAP uzfs ioctl implementation.
For command zfs listsnap <dataset>, output will be in json format as:

vitta@vitta-laptop:~/openebs/lzfs$ ./cmd/zfs/zfs listsnap pool1/ds0 | jq
{
  "name": "pool1/ds0",
  "snaplist": {
    "istgt1": null,
    "snap4": null,
    "istgt2": null,
    "snap1": null,
    "snap2": null,
    "istgt3": null,
    "snap3": null
  }
}

This will NOT list the snapshots created through 'zfs snapshot' until the zrepl is restarted. However, it lists the snapshots created through istgt iscsi target.

This command implementation reads the snapshot list from disk once and caches the list. This cache will be updated with snapshot name for every SNAP_CREATE command from istgt. This cache will be served for every trigger of this ioctl.

Working on testcases.

[vishnuitta]

Related PR to this in cstor: mayadata-io/cstor#295

[mittachaitu]

Can you update the PR description with updated sample JSON output?

[mittachaitu]

Can you update the PR description with updated sample JSON output?

Ok it was updated

[mittachaitu]

Looked more at usability angle and changes are good

[mynktl]

lgtm 👍

[mittachaitu]

Changes are good

[pawanpraka1]

looks good.

[pawanpraka1]

merged this, travis the failing as there is dependency on mayadata-io/cstor#295.