Automatic merge from submit-queue (batch tested with PRs 51902, 52718, 52687, 52137, 52697). 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>..
Refactoring and improvements for iSCSI and FC storage plugins
**What this PR does / why we need it**:
This PR makes following changes.
- Simplify volume tearDown path for iSCSI and FC using
util.UnmountPath().
- Log lastErr during iscsi connection
If iscsid fails to connect second portal, currently
the error is ignored silently. The lastErr should be
logged to find the root cause of problem.
- Remove iscsi plugin directory after iscsi connection
is successfully closed.
**Which issue this PR fixes** : No related issues.
**Special notes for your reviewer**:
/cc @rootfs @jsafrane
**Release note**:
```
NONE
```
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