Motivation for this change

A device doing IO to a slow device (USB HDD as an example) will end up "locking up" for a number of seconds due IO request queues filling up and new IO being relegated to the end of said queues.

WBT, introduced in 4.10 helps fix that.

Reading material:

• https://lwn.net/Articles/682582/
• https://lwn.net/Articles/704739/

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
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• 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.