fix(mm): handle concurrent page faults on same page

[Anekoique]

Problem

I encountered this problem when adapting to ROS2.
Multi-threaded applications crash with "No pages populated" error when
threads concurrently access the same lazy-loaded page in COW mappings.

Scenario:

1. Thread A triggers page fault on unmapped page
2. Page fault handler populates the page
3. Context switch to Thread B before Thread A resumes
4. Thread B faults on same page, finds it already mapped
5. populate() returns 0 (treated as failure) → segmentation fault

Root Cause

In both cow.rs and file.rs, the populate() functions only count
pages that need work (COW copy, permission upgrade, or new allocation),
but return 0 when a page is already correctly mapped by another thread.

Solution

In both backends, count already-mapped pages with correct permissions as
successful population.

[Copilot]

Pull request overview

This PR fixes a critical concurrency bug in the memory management module that caused multi-threaded applications to crash when concurrent page faults occurred on the same lazy-loaded page. The fix ensures that already-mapped pages with correct permissions are counted as successfully populated, preventing spurious "No pages populated" errors.

• Addresses race condition where Thread B faults on a page already mapped by Thread A
• Updates both COW and file-backed memory backends to count pre-mapped pages
• Prevents populate() from incorrectly returning 0 for already-correctly-mapped pages

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
modules/axmm/src/backend/cow.rs	Added check to count already-mapped pages with correct permissions in COW backend's populate function
modules/axmm/src/backend/file.rs	Added check to count already-mapped pages with correct permissions in file backend's populate function

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[Mivik]

Please update the docs of BackendOps::populate accordingly

[Anekoique]

Please update the docs of BackendOps::populate accordingly

DONE

[Mivik]

LGTM. Tested and it seems to work.