nixos-generate-config: add dm-snapshot module if LVM is detected

Without this, the system becomes unbootable if the user creates a LVM
snapshot and reboots.

Fixes https://github.com/NixOS/nixpkgs/issues/33646

(The same kind of problem was fixed in RHEL a few years back:
https://bugzilla.redhat.com/show_bug.cgi?id=1287940)