ng_socket: plug node reference leak

There is no need to reference node in its constructor, the framework
already granted us a reference.  This extraneous reference was later given
back on socket detach.  However, if node is destroyed before the socket,
the node was leaked.  This leak was harmless until VIMAGE, where
vnet_netgraph_uninit() would cycle through all nodes and shut them down
expecting them to not exist after the shutdown.

Fixes: 1928437497

sys/netgraph/ng_socket.c

@@ -506,7 +506,6 @@ ngc_attach(struct socket *so, int type, struct thread *td)
 
 	/* Link the node and the private data. */
 	NG_NODE_SET_PRIVATE(priv->node, priv);
-	NG_NODE_REF(priv->node);
 	priv->refs++;
 
 	return (0);
@@ -602,7 +601,6 @@ ng_socket_free_priv(struct ngsock *priv)
 
 		priv->node = NULL;
 		mtx_unlock(&priv->mtx);
-		NG_NODE_UNREF(node);
 		ng_rmnode_self(node);
 	} else
 		mtx_unlock(&priv->mtx);