Fix incorrect logic in GetDiskLun()

[EricMountain]

What type of PR is this?

What this PR does / why we need it:

See Go Operator Precedence.

package main

import (
	"fmt"
)

type DataDisk struct {
    Lun                     *int32                 `json:"lun,omitempty"`
}

func main() {
	var d DataDisk = DataDisk{}
	fmt.Printf("%v %v %d %v\n", false && false || false || true, d, d.Lun, d.Lun == nil)
}

https://play.golang.org/p/9r1y-5C99mr

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?

Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.: