guest-configs: enable Landlock LSM in 6.1 kernel configs

[Aakash456]

Enable CONFIG_SECURITY_LANDLOCK=y and prepend "landlock," to CONFIG_LSM in the 6.1 guest kernel configs for both x86_64 and aarch64 architectures.

Landlock is an unprivileged access control mechanism merged into mainline Linux in 5.13. It allows processes to restrict their own filesystem access without requiring privileges, complementing Firecracker's guest/host isolation boundary with fine-grained in-guest per-process sandboxing.

Landlock is not available in the 5.10 kernel configs (predates 5.13) so those are left unchanged.

The LSM ordering follows upstream Linux defaults, with "landlock" placed first in the initialization list.

Resolves #5997

Changes

Enable CONFIG_SECURITY_LANDLOCK=y and prepend "landlock," to
CONFIG_LSM in the 6.1 guest kernel configs for both x86_64 and
aarch64 architectures.

Landlock is an unprivileged access control mechanism merged into
mainline Linux in 5.13. It allows processes to restrict their own
filesystem access without requiring privileges, complementing
Firecracker's guest/host isolation boundary with fine-grained
in-guest per-process sandboxing. Workloads running inside the
microVM can use Landlock to enforce least-privilege filesystem
access on individual processes — for example agent runtimes,
build systems, or semi-trusted code — without requiring root,
containers, or additional host configuration.

The 5.10 kernel configs are intentionally left unchanged since
Landlock was not available until kernel 5.13.

The LSM ordering follows upstream Linux defaults, with "landlock"
placed first in the initialization list per the kernel's own
security/Kconfig defaults.

Testing

This change modifies kernel config files only. The actual kernel
build and validation requires Firecracker's devtool build
environment with resources/rebuild.sh. I have verified:

• CONFIG_SECURITY=y is present in both 6.1 configs (the only
  dependency for CONFIG_SECURITY_LANDLOCK)
• Landlock is not available in 5.10 (introduced in 5.13), so
  the 5.10 configs are correctly left unchanged
• The LSM string format matches existing conventions in the config

Resolves #5997

License Acceptance

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CONTRIBUTING.md.

PR Checklist

• I have read and understand CONTRIBUTING.md.
• I have run tools/devtool checkbuild --all to verify that the PR passes
  build checks on all supported architectures.
• I have run tools/devtool checkstyle to verify that the PR passes the
  automated style checks.
• I have described what is done in these changes, why they are needed, and
  how they are solving the problem in a clear and encompassing way.
• I have updated any relevant documentation (both in code and in the docs)
  in the PR.
• I have mentioned all user-facing changes in CHANGELOG.md.
• If a specific issue led to this PR, this PR closes the issue.
• When making API changes, I have followed the
  Runbook for Firecracker API changes.
• I have tested all new and changed functionalities in unit tests and/or
  integration tests.
• I have linked an issue to every new TODO.

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• This functionality cannot be added in rust-vmm.