fix(syscall):修复syslog的bug

[kaleidoscope416]

• 在 procfs 的 syslog syscall 处理路径中增加对 SYSLOG_ACTION_READ_ALL (action = 3) 的分发。
• 直接调用现有的 Kmsg::read_all 接口，未修改 Kmsg 的实现逻辑。
• 修复了 gvisor 测试中 Syslog.ReadAll 因未分发而返回 EINVAL 的问题。

[Copilot]

Pull Request Overview

This PR fixes a bug in the syslog syscall implementation where SYSLOG_ACTION_READ_ALL (action = 3) was not being handled. The fix adds the missing action handler to read all messages from the ring buffer, resolving a test failure in the gvisor test suite.

Key Changes:

• Added ReadAll variant to the SyslogAction enum and its dispatch logic
• Changed visibility of Kmsg::read_all from private to public
• Enabled the syslog_test in the gvisor test whitelist

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
kernel/src/filesystem/procfs/syscall.rs	Added ReadAll action variant and its handler mapping to kmsg_guard.read_all(buf)
kernel/src/filesystem/procfs/kmsg.rs	Changed read_all method visibility from private to public
user/apps/tests/syscall/gvisor/whitelist.txt	Enabled syslog_test in the test whitelist

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