io_uring: untie alloc_async_data and needs_async_data

All opcode handlers pretty well know whether they need async data or
not, and can skip testing for needs_async_data. The exception is rw
the generic path, but those test the flag by hand anyway. So, check the
flag and make io_alloc_async_data() allocating unconditionally.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>