io_uring: fix fs cleanup on cqe overflow

If completion queue overflow occurs, __io_cqring_fill_event() will
update req->cflags, which is in a union with req->work and happens to
be aliased to req->work.fs. Following io_free_req() ->
io_req_work_drop_env() may get a bunch of different problems (miscount
fs->users, segfault, etc) on cleaning @fs.
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -608,6 +608,7 @@ struct io_kiocb {
 	};
 
 	struct io_async_ctx		*io;
+	int				cflags;
 	bool				needs_fixed_file;
 	u8				opcode;
 
@@ -638,7 +639,6 @@ struct io_kiocb {
 			struct callback_head	task_work;
 			struct hlist_node	hash_node;
 			struct async_poll	*apoll;
-			int			cflags;
 		};
 		struct io_wq_work	work;
 	};