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/modules/services/x11/xserver.nix

@@ -666,6 +666,7 @@ in
     # The default max inotify watches is 8192.
     # Nowadays most apps require a good number of inotify watches,
     # the value below is used by default on several other distros.
+    boot.kernel.sysctl."fs.inotify.max_user_instances" = mkDefault 524288;
     boot.kernel.sysctl."fs.inotify.max_user_watches" = mkDefault 524288;
 
     systemd.defaultUnit = mkIf cfg.autorun "graphical.target";