linuxPackages.netatop: fix build with Linux 5.18
linuxPackages.netatop: fix build with Linux 5.18

With 5.18, implicit fallthrough is an error, and netatop hasn't caught
up yet.

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