Automatic merge from submit-queue (batch tested with PRs 45285, 45162)
mounter.go: format return err.
**What this PR does / why we need it**:
when an error returned is nil, it's preferred to explicitly return nil.
**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
```
f156f7a741