io_uring/rw: move ki_complete init into prep

Initialise ki_complete during request prep stage, we'll depend on it not
being reset during issue in the following patch.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/817624086bd5f0448b08c80623399919fda82f34.1739919038.git.asml.silence@gmail.com


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