Automatic merge from submit-queue (batch tested with PRs 60011, 59256, 59293, 60328, 60367). If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.

Move `storageclass` into  `storage` package under `plugin/pkg/admission`

We have two directories `storage` and `storageclass` under package `plugin/pkg/admission` and one subdir `storageobjectinuseprotection` of `storage` now.
- 1. Move `storageclass` into  `storage` being  `storage`'s sub-package since storageclass is one of the storage objects
- 2. fix golint failures

**What this PR does / why we need it**:

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

**Special notes for your reviewer**:

**Release note**:
```release-note
NONE
```