fix(process): 修复vfork实现并添加相关测试

[fslongjin]

• 修正vfork系统调用实现，使用正确的CLONE_VFORK | CLONE_VM标志
• 在execve中添加vfork完成通知机制
• 添加vfork相关测试用例到白名单

[Copilot]

Pull Request Overview

This PR fixes the vfork system call implementation to use the correct CLONE_VFORK | CLONE_VM flags and adds a completion notification mechanism in execve to wake the parent process. The changes ensure proper vfork semantics where the parent is blocked until the child calls execve or exits.

• Corrected vfork implementation to use CLONE_VFORK | CLONE_VM flags instead of empty flags
• Added vfork completion notification in execve before switching address spaces
• Added vfork_test to the test whitelist with specific test cases blocklisted

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
user/apps/tests/syscall/gvisor/whitelist.txt	Adds vfork_test to the enabled test suite
user/apps/tests/syscall/gvisor/blocklists/vfork_test	Creates blocklist for two specific vfork test cases that are not yet passing
kernel/src/process/syscall/sys_vfork.rs	Updates vfork implementation to use correct CLONE_VFORK and CLONE_VM flags
kernel/src/process/execve.rs	Adds vfork completion notification mechanism after successful execve

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