liveupdate: luo_file: allow keeping private_data from retrieve until finish

On Tue, Nov 10, 2026 at 06:58:52PM +0100, Pratyush Yadav wrote: >From: “Pratyush Yadav (Google)” pratyush@kernel.org > >The private_data field of struct liveupdate_file_op_args lets file >handlers k