fix(cstor): Remove our userspace implementation of io_getevents.

[sgielen]

This userspace implementation was added because it saves a system call if it would not return any new events at that point. However, to do this, it has to cast an intentionally opaque pointer, assume the structure of a kernel object, and read and write to it, which invokes undefined behaviour if the kernel structure is actually different. Concretely, the structure indeed seems to be different for a kernel I am running on arm64.

To ensure no undefined behaviour can happen, remove this optimization.

This PR is a continuation of arm64 stability fixes, see also:

• https://github.com/openebs/openebs/issues/3028
• https://github.com/openebs/cstor/pull/309

Checklist:

• [ ] Fixes #
• [X] PR Title follows the convention of <type>(<scope>): <subject>
• [x] Has the change log section been updated?
• [ ] Commit has unit tests
• [ ] Commit has integration tests
• [ ] (Optional) Are upgrade changes included in this PR? If not, mention the issue/PR to track:
• [ ] (Optional) If documentation changes are required, which issue on https://github.com/openebs/openebs-docs is used to track them:

[mynktl]

zrepl is stable with the existing changes as confirmed by the user in https://github.com/openebs/openebs/issues/3028#issuecomment-631708798, hence closing this PR, If required we can revisit this PR.

Thanks @sgielen !