fs: add IOCB flags related to passing back dio completions

fs: add IOCB flags related to passing back dio completions

Async dio completions generally happen from hard/soft IRQ context, which
means that users like iomap may need to defer some of the completion
handling to a workqueue. This is less efficient than having the original
issuer handle it, like we do for sync IO, and it adds latency to the
completions.

Add IOCB_DIO_DEFER, which the issuer can set if it is able to safely
punt these completions to a safe context. If the dio handler is aware
of this flag, assign a callback handler in kiocb->dio_complete and
associated data io kiocb->private. The issuer will then call this handler
with that data from task context.

No functional changes in this patch.

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