Prep for libnat20 kernel module

[werwurm]

Rename current to current_cdi, because it clashes with the Linux Kernel macro current.

[Copilot]

Pull request overview

Renames the n20_next_level_cdi_attest parameter from current to current_cdi to avoid a name clash with the Linux kernel current macro when preparing for a kernel-module build.

Changes:

• Rename n20_next_level_cdi_attest’s CDI input parameter from current to current_cdi in the C implementation.
• Update the public header prototype and its Doxygen parameter documentation to match.

Reviewed changes

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

File	Description
src/core/functionality.c	Renames the function parameter and updates the internal call to n20_derive_key.
include/nat20/functionality.h	Updates the exported API signature and Doxygen @param name to current_cdi.

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[github-actions]

LCOV of commit b6a418f during lcov-test-coverage-report #53

Summary coverage rate:
  lines......: 96.1% (2420 of 2517 lines)
  functions..: 99.5% (194 of 195 functions)
  branches...: 87.3% (1373 of 1573 branches)

Files changed coverage rate: n/a