nixos/firewall: fix inverted assertion for reverse path filtering
nixos/firewall: fix inverted assertion for reverse path filtering

Previously the assertion passed if the kernel had support OR the
filter was *enabled*. In the case of a kernel without support, the
`checkReversePath` option defaulted to false, and then failed the
assertion.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs