This fixes a bug in the stopping script for the xen-bridge service,
which caused the script to crash and fail to remove some
netfilter rules.

Motivation for this change

Currently, stopping the xen-bridge service results in the following error messages being logged by systemd, and two netfilter rules not being removed.

May 11 08:14:27 nixos systemd[1]: xen-bridge.service: Control process exited, code=exited status=1
May 11 08:14:27 nixos systemd[1]: Stopped Xen bridge.
May 11 08:14:27 nixos systemd[1]: xen-bridge.service: Unit entered failed state.
May 11 08:14:27 nixos systemd[1]: xen-bridge.service: Failed with result 'exit-code'.

This pull request corrects the commands that remove the netfilter rules.

Things done

• Tested using sandboxing
  (nix.useSandbox on NixOS,
  or option build-use-sandbox in nix.conf
  on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • Linux
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.