Fix filename for udev network interface rules

From http://www.freedesktop.org/wiki/Software/systemd/PredictableNetworkInterfaceNames/

You disable the assignment of fixed names, so that the unpredictable
kernel names are used again. For this, simply mask udev's rule file for
the default policy: ln -s /dev/null
/etc/udev/rules.d/80-net-setup-link.rules (since v209: this file was
called 80-net-name-slot.rules in release v197 through v208)

nixos/modules/services/hardware/udev.nix

@@ -87,8 +87,8 @@ let
         grep -l '\(RUN+\|IMPORT{program}\)="\(/usr\)\?/s\?bin' $i/*/udev/rules.d/* || true
       done
 
-      ${optionalString config.networking.usePredictableInterfaceNames ''
-        cp ${./80-net-name-slot.rules} $out/80-net-name-slot.rules
+      ${optionalString (! config.networking.usePredictableInterfaceNames) ''
+        cp ${./80-net-setup-link.rules} $out/80-net-setup-link.rules
       ''}
 
       # If auto-configuration is disabled, then remove