[BUG] Can't use longhorn with Generic ephemeral volumes about longhorn HOT 2 OPEN

Thank you for the report @mueller-tobias. I have confirmed the behavior and am investigating a fix. It looks like it is an unintended consequence of longhorn/longhorn-manager#2494 and its backports. longhorn/longhorn-manager#2659 and its backports mitigated the issue, so generic ephemeral volumes should only be broken in:

• Longhorn v1.6.0 (will be fixed in v1.6.1)
• Longhorn v1.5.4 (will be fixed in v1.5.5)

Longhorn v1.4.5 (if it is released) and v1.7.0 will contain both the breaking and mitigating changes (and so they will not be affected).

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Pre Ready-For-Testing Checklist

• Where is the reproduce steps/test steps documented?
  The reproduce steps/test steps are at: longhorn/longhorn-manager#2701 (comment).

• Is there a workaround for the issue? If so, where is it documented?
  There is no workaround outside of upgrading to a fixed version.

• Does the PR include the explanation for the fix or the feature?

• Does the PR include deployment change (YAML/Chart)? If so, where are the PRs for both YAML file and Chart?
  No, but I would like to add a new example that helps us test this feature: #8200.

• Have the backend code been merged (Manager, Engine, Instance Manager, BackupStore etc) (including backport-needed/*)?
  The PR is at: longhorn/longhorn-manager#2701.

• If labeled: require/doc Has the necessary document PR submitted or merged (including backport-needed/*)?
  There is a PR that adds a new example at: longhorn/website#885.

• If labeled: require/automation-e2e Has the end-to-end test plan been merged? Have QAs agreed on the automation test case? If only test case skeleton w/o implementation, have you created an implementation issue (including backport-needed/*)
  The automation skeleton PR is at: longhorn/longhorn-tests#1820.
  The issue of automation test case implementation is at :#8203.

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