longhorn/longhorn-manager: upgrade/v070to080/upgrade.go; 33 LoC

Found a possible issue in [longhorn/longhorn-manager](https://www.github.com/longhorn/longhorn-manager) at [upgrade/v070to080/upgrade.go](https://github.com/longhorn/longhorn-manager/blob/24300a52841e7f66b6568e05c327b2797df8abca/upgrade/v070to080/upgrade.go#L70-L102)

Below is the message reported by the analyzer for this snippet of code. Beware that the analyzer only reports the first
issue it finds, so please do not limit your consideration to the contents of the below message.

> reference to im is reassigned at line 74

[Click here to see the code in its original context.](https://github.com/longhorn/longhorn-manager/blob/24300a52841e7f66b6568e05c327b2797df8abca/upgrade/v070to080/upgrade.go#L70-L102)

Click here to show the 33 line(s) of Go which triggered the analyzer.

```go
for _, im := range imList.Items {
if im.Spec.Image != "" {
continue
}
im := &im
if types.ValidateEngineImageChecksumName(im.Spec.EngineImage) {
ei, err := lhClient.LonghornV1beta1().EngineImages(namespace).Get(im.Spec.EngineImage, metav1.GetOptions{})
if err != nil {
return errors.Wrapf(err, upgradeLogPrefix+"failed to find out the related engine image %v during the instance managers upgrade", im.Spec.EngineImage)
}
im.Spec.EngineImage = ei.Spec.Image
}
im.Spec.Image = im.Spec.EngineImage
node, exist := nodeMap[im.Spec.NodeID]
if !exist {
return fmt.Errorf(upgradeLogPrefix+"cannot to find node %v for instance manager %v during the instance manager upgrade", im.Spec.NodeID, im.Name)
}
metadata, err := meta.Accessor(im)
if err != nil {
return err
}
metadata.SetOwnerReferences(datastore.GetOwnerReferencesForNode(&node))
metadata.SetLabels(types.GetInstanceManagerLabels(im.Spec.NodeID, im.Spec.Image, im.Spec.Type))
if im, err = lhClient.LonghornV1beta1().InstanceManagers(namespace).Update(im); err != nil {
return errors.Wrapf(err, upgradeLogPrefix+"failed to update the spec for instance manager %v during the instance managers upgrade", im.Name)
}

im.Status.APIMinVersion = engineapi.IncompatibleInstanceManagerAPIVersion
im.Status.APIVersion = engineapi.IncompatibleInstanceManagerAPIVersion
if _, err = lhClient.LonghornV1beta1().InstanceManagers(namespace).UpdateStatus(im); err != nil {
return errors.Wrapf(err, upgradeLogPrefix+"failed to update the version status for instance manager %v during the instance managers upgrade", im.Name)
}
}

```

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commit ID: 24300a52841e7f66b6568e05c327b2797df8abca