io_uring: remove 'state' argument from io_{read,write} path

Since commit 09bb8394 we don't use the state argument for any sort
of on-stack caching in the io read and write path. Remove the stale
and unused argument from them, and bubble it up to __io_submit_sqe()
and down to io_prep_rw().

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