[netstack] Disable bridged endpoints

This is so the stack only sends packets through the bridge NIC and
not through the bridged endpoints NICs.

Change-Id: Iebc26e16c218ad80ceacb0c43f9f686fe325412f
diff --git a/src/connectivity/network/netstack/netstack.go b/src/connectivity/network/netstack/netstack.go
index 5ed6000..c55b61c 100644
--- a/src/connectivity/network/netstack/netstack.go
+++ b/src/connectivity/network/netstack/netstack.go
@@ -1032,6 +1032,9 @@
 				return nil, err
 			}
 		}
+		if err := ifs.ns.stack.DisableNIC(ifs.nicid); err != nil {
+			syslog.Errorf("error disabling NIC %d in stack.Stack while bridging endpoint: %s", ifs.nicid, err)
+		}
 		links = append(links, ifs.bridgeable)
 	}