linuxPackages: Revert accidental bump to 4.14

Regression introduced by e5c24ab.

I've added this for my WIP tests and forgot to reset them out of the
mentioned commits.

Signed-off-by: aszlig <aszlig@nix.build>

pkgs/top-level/all-packages.nix

@@ -12757,7 +12757,7 @@ with pkgs;
   });
 
   # The current default kernel / kernel modules.
-  linuxPackages = linuxPackages_4_14;
+  linuxPackages = linuxPackages_4_9;
   linux = linuxPackages.kernel;
 
   # Update this when adding the newest kernel major version!