linux: 4.9-rc5 -> 4.9-rc6

[NeQuissimus]

Motivation for this change

Update

Things done

• Tested using sandboxing
  (nix.useSandbox on NixOS,
  or option build-use-sandbox in nix.conf
  on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • Linux
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.

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[mention-bot]

@NeQuissimus, thanks for your PR! By analyzing the history of the files in this pull request, we identified @thoughtpolice, @7c6f434c and @Mathnerd314 to be potential reviewers.

[fpletz]

@NeQuissimus I don't think we need to backport testing kernels to stable. These shouldn't be used for stable/production environments anyway. (Also noted in #20413)

Furthermore, I don't think we need PRs for these simple version bumps. They only clutter the list of PRs and are of no use because they won't get a review (as it's not needed) and you end up merging them yourself anyway. That's why you should just push them to master.