Fix linter error (istio#1942)

Automatic merge from submit-queue.

Fix linter error

**What this PR does / why we need it**:
- Workaround vetshadow bug
- Add package alias for go-multierror

**Which issue this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close that issue when PR gets merged)*: fixes #

**Special notes for your reviewer**:

**Release note**:

```release-note
NONE
```

Author: manlinl

mixer/adapter/svcctrl/quotaprocessor.go

@@ -162,6 +162,7 @@ func getAllocatedQuotaAmount(args *adapter.QuotaArgs, quotaMetrics []*sc.MetricV
 
 // ProcessQuota calls Google ServiceControl client to allocate requested quota.
 // TODO(manlinl): Support handling multiple QuotaArgs once Mixer supports it. And implement retry on retriable error.
+// nolint: vetshadow
 func (p *quotaImpl) ProcessQuota(ctx context.Context,
 	instance *quota.Instance, args adapter.QuotaArgs) (adapter.QuotaResult, error) {
 	quotaCfg, found := p.quotaIndex[instance.Name]