io_uring: add IORING_OP_FADVISE

This adds support for doing fadvise through io_uring. We assume that
WILLNEED doesn't block, but that DONTNEED may block.

Reviewed-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

include/uapi/linux/io_uring.h

@@ -36,6 +36,7 @@ struct io_uring_sqe {
 		__u32		cancel_flags;
 		__u32		open_flags;
 		__u32		statx_flags;
+		__u32		fadvise_advice;
 	};
 	__u64	user_data;	/* data to be passed back at completion time */
 	union {
@@ -86,6 +87,7 @@ enum {
 	IORING_OP_STATX,
 	IORING_OP_READ,
 	IORING_OP_WRITE,
+	IORING_OP_FADVISE,
 
 	/* this goes last, obviously */
 	IORING_OP_LAST,