fix(mm): 修复madvise和msync系统调用中的边界处理问题

[fslongjin]

• 修复madvise长度参数未对齐时的处理逻辑，添加长度向上舍入和溢出检查
• 优化msync系统调用中的VMA查找逻辑，修复锁竞争和边界条件问题
• 修复UserMappings::find_nearest中的VMA查找逻辑错误
• 启用madvise测试

[Copilot]

Pull Request Overview

This pull request adds support for the madvise system call and fixes critical bugs in VMA (Virtual Memory Area) traversal logic. The changes improve memory management functionality and resolve issues related to memory mapping operations.

• Enables madvise_test in the test whitelist with specific blocked test cases
• Fixes the find_nearest VMA search logic to correctly find the next VMA after a given address
• Refactors sys_msync to release locks before calling find_nearest, preventing potential deadlocks
• Updates sys_madvise to properly handle unaligned length parameters and overflow conditions

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
user/apps/tests/syscall/gvisor/whitelist.txt	Adds madvise_test to the test suite
user/apps/tests/syscall/gvisor/blocklists/madvise_test	Defines blocked test cases for madvise (DontforkShared and DontforkAnonPrivate)
user/apps/tests/syscall/gvisor/blocklists/fork_test	Adds detailed comments explaining glibc error with COWSegment test
kernel/src/mm/ucontext.rs	Fixes find_nearest logic to search forward instead of backward
kernel/src/mm/syscall/sys_msync.rs	Refactors to release VMA lock before calling find_nearest, preventing deadlock
kernel/src/mm/syscall/sys_madvise.rs	Updates to handle unaligned length with proper overflow checking

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