Linux: Set passthru attr to contain full passthru

This fixes #1566, thanks @wizeman

Signed-off-by: Shea Levy <shea@shealevy.com>

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

@@ -120,6 +120,8 @@ let
     features = lib.fold (x: y: (x.features or {}) // y) features kernelPatches;
 
     meta = kernel.meta // extraMeta;
+
+    passthru = kernel.passthru // (removeAttrs passthru [ "passthru" ]);
   };
 
   nativeDrv = lib.addPassthru kernel.nativeDrv passthru;