io_uring: improve req fields comments

Move a misplaced comment about req->creds and add a line with
assumptions about req->link.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/1e51d1e6b1f3708c2d4127b4e371f9daa4c5f859.1648209006.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -935,10 +935,11 @@ struct io_kiocb {
 	struct async_poll		*apoll;
 	/* opcode allocated if it needs to store data for async defer */
 	void				*async_data;
-	/* custom credentials, valid IFF REQ_F_CREDS is set */
 	/* stores selected buf, valid IFF REQ_F_BUFFER_SELECTED is set */
 	struct io_buffer		*kbuf;
+	/* linked requests, IFF REQ_F_HARDLINK or REQ_F_LINK are set */
 	struct io_kiocb			*link;
+	/* custom credentials, valid IFF REQ_F_CREDS is set */
 	const struct cred		*creds;
 	struct io_wq_work		work;
 };