dfs:fix uninitialized 9pfs protocol lookup

[Aphlita]

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)

dfs_9pfs_mount()在查找9pfs protocol tag前声明了未初始化的p9p指针。如果_protocol_nodes为空，或者没有找到匹配的 tag，后续if (!p9p)会读取未初始化的局部指针，导致mount失败路径依赖不确定的栈值，而不是稳定返回错误。

你的解决方案是什么 (what is your solution)

将p9p初始化为RT_NULL,这样没有匹配protocol tag时,dfs_9pfs_mount()会稳定返回-RT_EINVAL。该修改只影响9pfs mount的错误路径，不改变成功路径行为。

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

• BSP:bsp/k210

• .config:本PR只修改9pfs mount错误路径中的局部变量初始化,无需修改.config

本地验证：

• git diff --check -- components/dfs/dfs_v1/filesystems/9pfs/dfs_9pfs.c
• cppcheck --enable=warning,style,performance,portability --quiet components/dfs/dfs_v1/filesystems/9pfs/dfs_9pfs.c

说明：本地尝试执行scons -j2验证bsp/k210，但本地环境缺少BSP配置的RISC-V工具链路径/opt/gnu-mcu-eclipse/riscv-none-gcc/8.2.0-2.1-20190425-1021/bin，因此未完成板级构建。

• action:https://github.com/Aphlita/rt-thread/actions/runs/24945700242

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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
• 如果是新增bsp, 已经添加ci检查到.github/ALL_BSP_COMPILE.json 详细请参考链接BSP自查

[github-actions]

👋 感谢您对 RT-Thread 的贡献！Thank you for your contribution to RT-Thread!

为确保代码符合 RT-Thread 的编码规范，请在你的仓库中执行以下步骤运行代码格式化工作流（如果格式化CI运行失败）。
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如有问题欢迎联系我们，再次感谢您的贡献！💐
If you have any questions, feel free to reach out. Thanks again for your contribution!

[github-actions]

📌 Code Review Assignment

🏷️ Tag: components

Reviewers: @Maihuanyi

Changed Files (Click to expand)

• components/dfs/dfs_v1/filesystems/9pfs/dfs_9pfs.c

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📊 Current Review Status (Last Updated: 2026-04-26 09:45 CST)

• ⌛ @Maihuanyi Pending Review

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