io_uring: control ->async_data with a REQ_F flag

->async_data is a slow path, so it won't matter much if we do the clean
up inside io_clean_op(). Moreover, in many cases it's allocated together
with setting one or more of IO_REQ_CLEAN_FLAGS flags, so it'd go through
io_clean_op() anyway.

Control ->async_data allocation with a new flag REQ_F_ASYNC_DATA, so we
can do all the maintainence under io_req_needs_clean() fast check.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/6892cf5883c459f36bda26f30ceb16742b20b84b.1633373302.git.asml.silence@gmail.com


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