io_uring: clear ->async_data as part of normal init

io_uring: clear ->async_data as part of normal init

Opcode handlers like POLL_ADD will use ->async_data as the pointer for
double poll handling, which is a bit different than the usual case
where it's strictly gated by the REQ_F_ASYNC_DATA flag. Be a bit more
proactive in handling ->async_data, and clear it to NULL as part of
regular init. Init is touching that cacheline anyway, so might as well
clear it.

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