io_uring: support to inject result for NOP

Support to inject result for NOP so that we can inject failure from
userspace. It is very helpful for covering failure handling code in
io_uring core change.

With nop flags, it becomes possible to add more test features on NOP in
future.

Suggested-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20240510035031.78874-3-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

include/uapi/linux/io_uring.h

@@ -72,6 +72,7 @@ struct io_uring_sqe {
 		__u32		waitid_flags;
 		__u32		futex_flags;
 		__u32		install_fd_flags;
+		__u32		nop_flags;
 	};
 	__u64	user_data;	/* data to be passed back at completion time */
 	/* pack this to avoid bogus arm OABI complaints */
@@ -407,6 +408,13 @@ enum io_uring_msg_ring_flags {
  */
 #define IORING_FIXED_FD_NO_CLOEXEC	(1U << 0)
 
+/*
+ * IORING_OP_NOP flags (sqe->nop_flags)
+ *
+ * IORING_NOP_INJECT_RESULT	Inject result from sqe->result
+ */
+#define IORING_NOP_INJECT_RESULT	(1U << 0)
+
 /*
  * IO completion data structure (Completion Queue Entry)
  */