nixos/wireguard: fix mtu after switching netns.

mtu is set after switching netns and thus the new netns should be used.

nixos/modules/services/networking/wireguard.nix

···
 
           ${ipPreMove} link add dev "${name}" type wireguard
           ${optionalString (values.interfaceNamespace != null && values.interfaceNamespace != values.socketNamespace) ''${ipPreMove} link set "${name}" netns "${ns}"''}
-          ${optionalString (values.mtu != null) ''${ipPreMove} link set "${name}" mtu ${toString values.mtu}''}
+          ${optionalString (values.mtu != null) ''${ipPostMove} link set "${name}" mtu ${toString values.mtu}''}
 
           ${concatMapStringsSep "\n" (ip:
             ''${ipPostMove} address add "${ip}" dev "${name}"''

nixos/tests/wireguard/namespaces.nix

···
         preSetup = ''
           ip netns add ${interfaceNamespace}
         '';
+        mtu = 1280;
         inherit interfaceNamespace;
       };
     };