linux_testing_hardened: fix modDirVersion

and bring the linux-testing expression closer to the other versions.

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

@@ -1,10 +1,14 @@
-{ stdenv, buildPackages, fetchurl, perl, buildLinux, libelf, utillinux, ... } @ args:
+{ stdenv, buildPackages, fetchurl, perl, buildLinux, modDirVersionArg ? null, ... } @ args:
+
+with stdenv.lib;
 
 buildLinux (args // rec {
   version = "5.2-rc6";
-  modDirVersion = "5.2.0-rc6";
   extraMeta.branch = "5.2";
 
+  # modDirVersion needs to be x.y.z, will always add .0
+  modDirVersion = if (modDirVersionArg == null) then builtins.replaceStrings ["-"] [".0-"] version else modDirVersionArg;
+
   src = fetchurl {
     url = "https://git.kernel.org/torvalds/t/linux-${version}.tar.gz";
     sha256 = "15lwy9596v5sw8c6mhpl9ilfcmm39snvvyajg08ycsg61i2s58v0";