[DECNET]: Only use local routers

The attached patch makes DECnet routing only use routers from the same
area - rather than the highest rated router seen.

In theory there should not be an out-of-area router on a local network
but some networks are bridged rather than properly routed. VMS seems
to behave similarly: if I bring up a VMS node with no router then it
can't see anything else on the global network.
Signed-off-by: Patrick Caulfield <patrick@tykepenguin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/decnet/dn_neigh.c

@@ -408,12 +408,15 @@ int dn_neigh_router_hello(struct sk_buff *skb)
 			}
 		}
 
+		/* Only use routers in our area */
+		if ((dn_ntohs(src)>>10) == dn_ntohs((decnet_address)>>10)) {
 			if (!dn_db->router) {
 				dn_db->router = neigh_clone(neigh);
 			} else {
 				if (msg->priority > ((struct dn_neigh *)dn_db->router)->priority)
 					neigh_release(xchg(&dn_db->router, neigh_clone(neigh)));
 			}
+		}
 		write_unlock(&neigh->lock);
 		neigh_release(neigh);
 	}