io_uring/rw: add write support for IOCB_DIO_DEFER

io_uring/rw: add write support for IOCB_DIO_DEFER

If the filesystem dio handler understands IOCB_DIO_DEFER, we'll get
a kiocb->ki_complete() callback with kiocb->dio_complete set. In that
case, rather than complete the IO directly through task_work, queue
up an intermediate task_work handler that first processes this
callback and then immediately completes the request.

For XFS, this avoids a punt through a workqueue, which is a lot less
efficient and adds latency to lower queue depth (or sync) O_DIRECT
writes.

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