fix: [disk-encrypt] unencrypt part error

[itsXuSt]

The seek pos is greater than part size.
add limit for this.

Change-Id: I07839506f171858152ce2865f812714769023d9e

[deepin-ci-robot]

deepin pr auto review

关键摘要:

• 在修改后的代码中，当partSize % kStepSize == 0时，mvCount的值会减1，这可能是一个逻辑错误，因为原代码中当partSize % kStepSize不为0时才需要增加1。
• 新增的if (currMovedIndex * kStepSize >= partSize)条件判断中，如果currMovedIndex * kStepSize大于或等于partSize，则会打印警告信息并调用clearMem()函数。这个逻辑可能不正确，因为clearMem()函数没有被在提交中定义或引入。
• 在blockFile.seek(currMovedIndex * kStepSize)语句中，如果seek pos failed!的信息被打印，但没有相应的错误处理或恢复机制，可能会导致程序异常终止。

是否建议立即修改:
是

[deepin-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: itsXuSt, Johnson-zs

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[itsXuSt]

/merge