selftests: ublk: specify io_cmd_buf pointer type

Matching the ublk driver, change the type of io_cmd_buf from char * to
struct ublksrv_io_desc *.

Signed-off-by: Caleb Sander Mateos <csander@purestorage.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Acked-by: Shuah Khan <skhan@linuxfoundation.org>
Link: https://lore.kernel.org/r/20250328194230.2726862-3-csander@purestorage.com


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