io_uring: add generic command payload type to struct io_kiocb

Each opcode generally has a command structure in io_kiocb which it can
use to store data associated with that request.

In preparation for having the core layer not know about what's inside
these fields, add a generic io_cmd_data type and put in the union as
well.
Signed-off-by: Jens Axboe <axboe@kernel.dk>

io_uring/io_uring.c

@@ -954,14 +954,27 @@ enum {
 };
 
 /*
- * NOTE! Each of the iocb union members has the file pointer
+ * Each request type overlays its private data structure on top of this one.
+ * They must not exceed this one in size.
+ */
+struct io_cmd_data {
+	struct file		*file;
+	/* each command gets 56 bytes of data */
+	__u8			data[56];
+};
+
+#define io_kiocb_to_cmd(req)	((void *) &(req)->cmd)
+
+struct io_kiocb {
+	union {
+		/*
+		 * NOTE! Each of the io_kiocb union members has the file pointer
 		 * as the first entry in their struct definition. So you can
 		 * access the file pointer through any of the sub-structs,
 		 * or directly as just 'file' in this struct.
 		 */
-struct io_kiocb {
-	union {
 		struct file		*file;
+		struct io_cmd_data	cmd;
 		struct io_rw		rw;
 		struct io_poll_iocb	poll;
 		struct io_poll_update	poll_update;