[ipv4] Ignore non-open net devices when performing routing

We do not discard routing table entries when closing an interface.  It
is plausible that multiple interfaces may be on the same physical
network; if so, then we may end up in a situation whereby outbound
packets attempt to route via a closed interface.

Fix by ignoring non-open net devices in ipv4_route().

src/net/ipv4.c

@@ -118,6 +118,8 @@ static struct ipv4_miniroute * ipv4_route ( struct in_addr *dest ) {
 
 	/* Find first usable route in routing table */
 	list_for_each_entry ( miniroute, &ipv4_miniroutes, list ) {
+		if ( ! ( miniroute->netdev->state & NETDEV_OPEN ) )
+			continue;
 		local = ( ( ( dest->s_addr ^ miniroute->address.s_addr )
 			    & miniroute->netmask.s_addr ) == 0 );
 		has_gw = ( miniroute->gateway.s_addr );