needs_async_data controls allocation of async_data, and used in two
cases. 1) when async setup requires it (by io_req_prep_async() or
handler themselves), and 2) when op always needs additional space to
operate, like timeouts do.

Opcode preps already don't bother about the second case and do
allocation unconditionally, restrict needs_async_data to the first case
only and rename it into needs_async_setup.
Signed-off-by: NPavel Begunkov <asml.silence@gmail.com>
[axboe: update for IOPOLL fix]
Signed-off-by: NJens Axboe <axboe@kernel.dk>