io_uring: explain io_wq_work::cancel_seq placement

Add a comment on why we keep ->cancel_seq in struct io_wq_work instead
of struct io_kiocb despite it needed only by io_uring but not io-wq.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/988e87eec9dc700b5dae933df3aefef303502f6c.1655310733.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

io_uring/io-wq.h

@@ -155,6 +155,7 @@ struct io_wq_work_node *wq_stack_extract(struct io_wq_work_node *stack)
 struct io_wq_work {
 	struct io_wq_work_node list;
 	unsigned flags;
+	/* place it here instead of io_kiocb as it fills padding and saves 4B */
 	int cancel_seq;
 };