[NETLINK]: Fix mc_list operations

While doing the 2.4 version of the netlink patch, I noticed a bug
in my mc_list code.  First of all it wasn't holding the table lock
when adding nodes to the list.  It also didn't take the node off
the list when multicast is switched off.
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/netlink/af_netlink.c

@@ -471,9 +471,16 @@ static int netlink_bind(struct socket *sock, struct sockaddr *addr, int addr_len
 			return err;
 	}
 
-	nlk->groups = nladdr->nl_groups;
-	if (nladdr->nl_groups)
+	if (!nladdr->nl_groups && !nlk->groups)
+		return 0;
+
+	netlink_table_grab();
+	if (nlk->groups && !nladdr->nl_groups)
+		__sk_del_bind_node(sk);
+	else if (!nlk->groups && nladdr->nl_groups)
 		sk_add_bind_node(sk, &nl_table[sk->sk_protocol].mc_list);
+	nlk->groups = nladdr->nl_groups;
+	netlink_table_ungrab();
 
 	return 0;
 }