Fix calculate the part free size

[jianghushinian]

I think there might be a little bit of a problem with calculate the part free size, my disk partition looks like this:

$ sudo parted /dev/sdb unit b print free --script -m
BYT;
/dev/sdb:10737418240B:scsi:512:512:gpt:ATA VBOX HARDDISK:;
1:17408B:1048575B:1031168B:free;
1:1048576B:10485759B:9437184B::test-device:;
2:10485760B:1084227583B:1073741824B:ext4:301d32a8-b9b0-4c60-9585-aa4ee7364e2b:;
3:1084227584B:4305453055B:3221225472B:ext4:f9ee1a0a-e2b9-498f-9d30-97ece3195e81:;
1:4305453056B:7526678527B:3221225472B:free;
5:7526678528B:8600420351B:1073741824B:ext4:ab6a7db4-cc74-4e72-b6aa-f5a09b0ef58b:;
1:8600420352B:10737401343B:2136980992B:free;

In fact, calculate the part free size: 3221225472B = 7526678527B - 4305453056B + 1B.

https://github.com/openebs/device-localpv/blob/cc23b1acdfe9195bc4f412805909deabf46da9b1/pkg/device/device-util.go#L292-L306

Then, in findBestPart the code judgment of if tmp.SizeMiB > partSize ignores the case of equal capacity.

https://github.com/openebs/device-localpv/blob/cc23b1acdfe9195bc4f412805909deabf46da9b1/pkg/device/device-util.go#L224-L251

These two problems can result in a bug, i cannot apply for another 3Gi PVC:

➜ kubectl get pvc csi-devicepv1-hello1-1 -o yaml
apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  creationTimestamp: "2022-07-22T07:18:07Z"
  finalizers:
  - kubernetes.io/pvc-protection
  labels:
    app: hello1
  name: csi-devicepv1-hello1-1
  namespace: default
  resourceVersion: "10121"
  uid: df90aafe-504d-40b8-bb5d-0c85458a2e68
spec:
  accessModes:
  - ReadWriteOnce
  resources:
    requests:
      storage: 3Gi
  storageClassName: openebs-device-sc
  volumeMode: Filesystem
status:
  phase: Pending

The PVC will always be in a Pending state.

This pull request is used to fix this bug: #57.

[jianghushinian]

@kmova @iyashu PTAL.