【修改】tmpfs下读写错误

[zhuzhuzhus]

拉取/合并请求描述：(PR description)

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为什么提交这份PR (why to submit this PR)

在验证根文件系统可行性的时候发现在tmpfs文件系统下读写文件系统会崩溃

你的解决方案是什么 (what is your solution)

tmp文件系统读写过程中，执行dfs_tmpfs_read时，由于file->vnode->size是无符号类型，所以在进行if(count < file->vnode->size - ops)这一步时会导致如果size小于ops时，减出来的数不是负数，而是一个非常大的正数，这样会导致即使文件没有内容读了。也会执行memcpy(buf, &(d_file->data[*pos]), length);这个函数，进而导致内存报错，所以在该函数中强制转换了一部分数据类型，目前运行正常，我编写了一个utest在rt-thread/examples/utest/testcases/dfs/tmpfs目录下，目前已经可以正常通过测试。

请提供验证的bsp和config (provide the config and bsp)

• BSP:rt-thread/bsp/qemu-virt64-aarch64

• .config:

• action:

]

当前拉取/合并请求的状态 Intent for your PR

必须选择一项 Choose one (Mandatory):

• 本拉取/合并请求是一个草稿版本 This PR is for a code-review and is intended to get feedback
• 本拉取/合并请求是一个成熟版本 This PR is mature, and ready to be integrated into the repo

代码质量 Code Quality：

我在这个拉取/合并请求中已经考虑了 As part of this pull request, I've considered the following:

• 已经仔细查看过代码改动的对比 Already check the difference between PR and old code
• 代码风格正确，包括缩进空格，命名及其他风格 Style guide is adhered to, including spacing, naming and other styles
• 没有垃圾代码，代码尽量精简，不包含#if 0代码，不包含已经被注释了的代码 All redundant code is removed and cleaned up
• 所有变更均有原因及合理的，并且不会影响到其他软件组件代码或BSP All modifications are justified and not affect other components or BSP
• 对难懂代码均提供对应的注释 I've commented appropriately where code is tricky
• 代码是高质量的 Code in this PR is of high quality
• 已经使用formatting 等源码格式化工具确保格式符合RT-Thread代码规范 This PR complies with RT-Thread code specification

[CLAassistant]

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1 out of 2 committers have signed the CLA.

✅ rcitachi
❌ zhujiale

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[BernardXiong]

please agree CLA and check the conflicting files.