io_uring: enable READ/WRITE to use deferred completions

io_uring: enable READ/WRITE to use deferred completions

A bit more surgery required here, as completions are generally done
through the kiocb->ki_complete() callback, even if they complete inline.
This enables the regular read/write path to use the io_comp_state
logic to batch inline completions.

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