selftests: ublk: prepare for supporting stripe target

- pass 'truct dev_ctx *ctx' to target init function

- add 'private_data' to 'struct ublk_dev' for storing target specific data

- add 'private_data' to 'struct ublk_io' for storing per-IO data

- add 'tgt_ios' to 'struct ublk_io' for counting how many io_uring ios
for handling the current io command

- add helper ublk_get_io() for supporting stripe target

- add two helpers for simplifying target io handling

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250322093218.431419-6-ming.lei@redhat.com


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