fix: Fix DDO pledge math

[simlecode]

关联的Issues (Related Issues)

https://github.com/filecoin-project/lotus/pull/12341/files

改动 (Proposed Changes)

调整 DDO 订单计算质押的逻辑

附注 (Additional Info)

自查清单 (Checklist)

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• 符合 Venus 项目管理规范中关于 PR 的相关标准 / The PR follows the PR standards set out in the Venus project management guidelines
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• 包含相关的的测试用例或者不需要新增测试用例 / This PR has tests for new functionality or change in behaviour or not need to add new tests.
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