Revert removal of `linux_testing_bcachefs` from "linux: remove versions unmaintained upstream"

This reverts parts of commit 298e170b36b79f4c6b19816a7abbf8392a490840.

pkgs/os-specific/linux/kernel/linux-testing-bcachefs.nix

···
+{ stdenv, buildPackages, hostPlatform, fetchgit, perl, buildLinux, ... } @ args:
+
+buildLinux (args // rec {
+  version = "4.15.2018.02.09";
+  modDirVersion = "4.15.0";
+  extraMeta.branch = "master";
+  extraMeta.maintainers = [ stdenv.lib.maintainers.davidak ];
+
+  src = fetchgit {
+    url = "https://evilpiepirate.org/git/bcachefs.git";
+    rev = "4506cd5ead31209a6a646c2412cbc7be735ebda4";
+    sha256 = "0fcyf3y27k2lga5na4dhdyc47br840gkqynv8gix297pqxgidrib";
+  };
+
+  extraConfig = ''
+    BCACHEFS_FS m
+  '';
+
+  # Should the testing kernels ever be built on Hydra?
+  extraMeta.hydraPlatforms = [];
+
+} // (args.argsOverride or {}))

pkgs/top-level/all-packages.nix

···
       ];
   };
 
+  linux_testing_bcachefs = callPackage ../os-specific/linux/kernel/linux-testing-bcachefs.nix {
+    kernelPatches =
+      [ kernelPatches.bridge_stp_helper
+        kernelPatches.modinst_arg_list_too_long
+      ]
+      ++ lib.optionals ((platform.kernelArch or null) == "mips")
+      [ kernelPatches.mips_fpureg_emu
+        kernelPatches.mips_fpu_sigill
+        kernelPatches.mips_ext3_n32
+      ];
+  };
+
   linux_riscv = callPackage ../os-specific/linux/kernel/linux-riscv.nix {
     kernelPatches = [
       kernelPatches.bridge_stp_helper
···
       };
     };
     in tinyLinuxPackages.kernel;
+
+  # Build a kernel with bcachefs module
+  linuxPackages_testing_bcachefs = recurseIntoAttrs (linuxPackagesFor pkgs.linux_testing_bcachefs);
 
   # Build a kernel for Xen dom0
   linuxPackages_xen_dom0 = recurseIntoAttrs (linuxPackagesFor (pkgs.linux.override { features.xen_dom0=true; }));