kernel-common: Add ZBUD

[NeQuissimus]

Motivation for this change

Fix #18533

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
  • OS X
  • 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.

I built a NixOS generation based on master with this change and the warnings mentioned in #18533 no longer appear.

---

Ping @abbradar

[mention-bot]

@NeQuissimus, thanks for your PR! By analyzing the annotation information on this pull request, we identified @edolstra, @wkennington and @zohl to be potential reviewers

[abbradar]

Can you also make ZSMALLOC m? It appears that ZBUD was made the default back in 2013, so we can make ZSMALLOC a module to save space.

[dezgeg]

This sounds a bit like it's working around an upstream problem, either the kconfig dependencies should prevent an invalid configuration from being selected, or the code should cope with the default being a module.

[abbradar]

Hm, I've approved this prematurely because we need to test one more thing -- can you try to build linux_3_10 to see if it's okay? Maybe we need to make ZBUD = ?y or something.

[NeQuissimus]

Will kick off a kernel build in a minute...

[NeQuissimus]

I built 3.10, 3.12, 3.18 and 4.7. They all seem to build correctly now.

ZSMALLOC does not like being a module before 3.18

[NeQuissimus]

I think the Travis failures are timeouts, not actual errors

[rasendubi]

@abbradar can this be merged?