Add copy_wp() method for UFFDIO_COPY with UFFDIO_COPY_MODE_WP

[vtta]

• Add copy_wp() method that combines UFFDIO_COPY with UFFDIO_COPY_MODE_WP to resolve a page fault and enable write-protection tracking in a single ioctl call
• Useful for dirty-page tracking scenarios where you want to resolve a missing-page fault while simultaneously enabling write-protection monitoring, avoiding a separate write_protect()
  call
• Add test_copy_wp test validating the full missing-fault → copy+WP → write-protect-fault → remove-WP flow

Test plan

• cargo build --features linux5_7 compiles
• cargo test --features linux5_7 — all 9 tests pass (5 unit, 1 integration, 3 doc-tests) on Linux 6.8
• New test_copy_wp verifies:
  1. Missing fault resolved with copy_wp() copies source data and sets WP in one ioctl
  2. Subsequent write triggers a write-protect fault
  3. After removing WP, the write lands successfully