lkl: downgrade to 5.15

[yu-re-ka]

Description of changes

This should fix nix build -f nixos/release-combined.nix nixos.ova and thus unblock the unstable channel

I bisected lkl to find out that >=5.16 breaks the cptofs thing, but 5.15 still works fine for nftables checkRuleset purposes.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 23.05 Release Notes (or backporting 22.11 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

[vcunat]

For reference, the ova build and unblocking need(ed) further fixes, like #219463

[Mic92]

I recently did some upstream work. I didn't encountered a hang: lkl/linux#534 but fixed some other filesystem unmount issue. Maybe worth to give it a retry.

[Mic92]

Still seems to hang... One feature I added to cptofs recently is ownership..., which would be also interesting for make-disk-image.