ospfd: Change struct ospf_path *oi to ifindex.

* global: In struct ospf_path, change struct ospf_interface *oi to int
  ifindex.  It is unsafe to reference *oi as an ospf interface can be
  deleted under your feet. Use a weak reference instead.
diff --git a/ospfd/ospf_route.h b/ospfd/ospf_route.h
index 0d37436..17ab68e 100644
--- a/ospfd/ospf_route.h
+++ b/ospfd/ospf_route.h
@@ -39,7 +39,7 @@
 {
   struct in_addr nexthop;
   struct in_addr adv_router;
-  struct ospf_interface *oi;
+  unsigned int ifindex;
 };
 
 /* Below is the structure linked to every