io_uring: add IOSQE_ASYNC

io_uring defaults to always doing inline submissions, if at all
possible. But for larger copies, even if the data is fully cached, that
can take a long time. Add an IOSQE_ASYNC flag that the application can
set on the SQE - if set, it'll ensure that we always go async for those
kinds of requests. Use the io-wq IO_WQ_WORK_CONCURRENT flag to ensure we
get the concurrency we desire for this case.

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

include/uapi/linux/io_uring.h

@@ -51,6 +51,7 @@ struct io_uring_sqe {
 #define IOSQE_IO_DRAIN		(1U << 1)	/* issue after inflight IO */
 #define IOSQE_IO_LINK		(1U << 2)	/* links next sqe */
 #define IOSQE_IO_HARDLINK	(1U << 3)	/* like LINK, but stronger */
+#define IOSQE_ASYNC		(1U << 4)	/* always go async */
 
 /*
  * io_uring_setup() flags