refactor(uffd): store pageSize on struct

[ValentaTomas]

Summary

• Store pageSize on the Userfaultfd struct instead of retrieving it from mapping functions on every fault.
• Drop the pagesize return value from Mapping.GetOffset and Mapping.GetHostVirtAddr; callers now read u.pageSize.
• Drop the pagesize parameter from faultPage.
• Update mapping offset tests for the new signatures.

All regions in a mapping are already validated to share the same page size in NewUserfaultfdFromFd, so a single stored value is sufficient.

Pure refactor, no behavior change.

Split out of this PR

• Prefault / directDataSource extraction into prefault.go → refactor(uffd): extract Prefault to its own file #2441
• safeInvoke helper extraction → refactor(uffd): extract safeInvoke helper #2442

Test plan

• go build ./... passes
• go vet ./pkg/sandbox/uffd/... clean
• golangci-lint run ./pkg/sandbox/uffd/... clean
• Existing mapping tests updated

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PR Summary

Low Risk
Low risk refactor that primarily changes function signatures and threads a cached pageSize through the userfaultfd fault/prefault paths. Main risk is missed call-site updates leading to incorrect page sizing or compile-time breakage.

Overview
Stores a single pageSize on Userfaultfd (validated once at construction) and refactors the UFFD faulting/prefaulting path to use it, removing per-call page-size plumbing. As part of this, Mapping.GetOffset and Mapping.GetHostVirtAddr drop their page-size return values, faultPage no longer accepts a pagesize parameter, and the mapping offset tests are updated to the new signatures.

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