io_uring: return boolean value for io_alloc_async_data

boolean value is good enough for io_alloc_async_data.

Signed-off-by: Hao Xu <haoxu@linux.alibaba.com>
Link: https://lore.kernel.org/r/20210922101522.9179-1-haoxu@linux.alibaba.com


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