ublk: store auto buffer register data into `struct ublk_io`

We can share space of `io->addr` for storing auto buffer register data
and user space buffer address.

So store auto buffer register data into `struct ublk_io`.

Prepare for supporting batch IO in which many ublk IOs share single
uring_cmd, so we can't store auto buffer register data into uring_cmd
pdu.

Reviewed-by: Caleb Sander Mateos <csander@purestorage.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250713143415.2857561-8-ming.lei@redhat.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>