io_uring: simplify io_resubmit_prep()

If not for async_data NULL check, io_resubmit_prep() is already an rw
specific version of io_req_prep_async(), but slower because 1) it always
goes through io_import_iovec() even if following io_setup_async_rw() the
result 2) instead of initialising iovec/iter in-place it does it
on-stack and then copies with io_setup_async_rw().

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