[DLM] Telnet to port 21064 can stop all lockspaces

This patch fixes Red Hat bz#245892

Opening a tcp connection from a cluster member to another cluster member
targeting the dlm port it is enough to stop every dlm operation in the cluster.
This means that GFS and rgmanager will hang.
Signed-Off-By: NPatrick Caulfield <pcaulfie@redhat.com>
Signed-off-by: NSteven Whitehouse <swhiteho@redhat.com>

fs/dlm/lowcomms.c

@@ -720,12 +720,18 @@ static int tcp_accept_from_sock(struct connection *con)
 			INIT_WORK(&othercon->rwork, process_recv_sockets);
 			set_bit(CF_IS_OTHERCON, &othercon->flags);
 			newcon->othercon = othercon;
-		}
 			othercon->sock = newsock;
 			newsock->sk->sk_user_data = othercon;
 			add_sock(newsock, othercon);
 			addcon = othercon;
 		}
+		else {
+			printk("Extra connection from node %d attempted\n", nodeid);
+			result = -EAGAIN;
+			up_write(&newcon->sock_sem);
+			goto accept_err;
+		}
+	}
 	else {
 		newsock->sk->sk_user_data = newcon;
 		newcon->rx_action = receive_from_sock;