zfs: Sync pools on shutdown.

nixos/modules/tasks/filesystems/zfs.nix

@@ -466,6 +466,11 @@ in
         '') rootPools));
       };
 
+      systemd.shutdownRamfs.contents."/etc/systemd/system-shutdown/zpool".source = pkgs.writeShellScript "zpool-sync-shutdown" ''
+        exec ${cfgZfs.package}/bin/zpool sync
+      '';
+      systemd.shutdownRamfs.storePaths = ["${cfgZfs.package}/bin/zpool"];
+
       # TODO FIXME See https://github.com/NixOS/nixpkgs/pull/99386#issuecomment-798813567. To not break people's bootloader and as probably not everybody would read release notes that thoroughly add inSystem.
       boot.loader.grub = mkIf (inInitrd || inSystem) {
         zfsSupport = true;