feat(fs): add Linux-style errseq writeback reporting

[fslongjin]

Summary

Implements Linux-style errseq_t writeback error reporting for DragonOS page cache and sync paths.

• Add a reusable atomic ErrSeq primitive matching Linux's sequence/error/seen-bit model.
• Replace ad-hoc SpinLock<seq,error> writeback error state in PageCache and superblock state.
• Track page-cache and superblock errseq cursors per open file description, protected by a file-local mutex.
• Wire errseq consumption into fsync, fdatasync, msync(MS_SYNC), sync_file_range, and syncfs paths.
• Add /sys/kernel/debug/errseq/selftest plus a dunitest wrapper.
• Add errseq-设计.md with the Linux/DragonOS architecture analysis and review notes.

Fixes #1909.

Validation

Host-side:

• make fmt
• make kernel
• make -C user/apps/tests/dunitest build-suites

DragonOS guest via make run-nographic after refreshing the disk image:

• ./errseq_selftest_test - 2/2 passed
• ./fdatasync_test - 7/7 passed
• ./sync_file_range_test - 4/4 passed
• ./syncfs_semantics_test - 8/8 passed
• ./mlock_semantics_test - 10 passed, 1 skipped (MlockAll.FutureAppliesToBrkGrowth, environment cannot grow brk by one page without mlockall)

[fslongjin]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. You're on a roll.

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