nixos/bcachefs: Use kernel 6.18

Ideally, the module does not set a non-LTS kernel by default, which we
have been able to do since 6.18 was labeled LTS. This should have been
done before 6.19 became `latest` to avoid having people upgraded to
6.19 and then downgraded to 6.18, but we just missed that. Shouldn't
be a problem if we rectify it quickly.

nixos/modules/tasks/filesystems/bcachefs.nix

@@ -242,8 +242,8 @@ in
           }
         ];
 
-        # Bcachefs upstream recommends using the latest kernel
-        boot.kernelPackages = lib.mkDefault pkgs.linuxPackages_latest;
+        # Use LTS that bcachefs is compatible with.
+        boot.kernelPackages = lib.mkDefault pkgs.linuxPackages_6_18;
 
         # needed for systemd-remount-fs
         system.fsPackages = [ cfg.package ];