nixos/xserver: set fs.inotify.max_user_instances too
nixos/xserver: set fs.inotify.max_user_instances too

A too low number of inotify user instances causes similar problems as
max_user_watches. Without this, my workstation keeps running into things
like this:

$ sudo systemctl restart display-manager.service
Failed to allocate directory watch: Too many open files

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