io_uring: refactor out send/recv async setup

IORING_OP_[SEND,RECV] don't need async setup neither will get into
io_req_prep_async(). Remove them from io_req_prep_async() and remove
needs_async_data checks from the related setup functions.

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