io_uring: zero ref_node after killing it

After a rsrc/files reference node's refs are killed, it must never be
used. And that's how it works, it either assigns a new node or kills the
whole data table.

Let's explicitly NULL it, that shouldn't be necessary, but if something
would go wrong I'd rather catch a NULL dereference to using a dangling
pointer.
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -7299,6 +7299,7 @@ static void io_sqe_rsrc_kill_node(struct io_ring_ctx *ctx, struct fixed_rsrc_dat
 
 	io_rsrc_ref_lock(ctx);
 	ref_node = data->node;
+	data->node = NULL;
 	io_rsrc_ref_unlock(ctx);
 	if (ref_node)
 		percpu_ref_kill(&ref_node->refs);