ospfd: Optimize and improve SPF nexthop calculation

Maintain router LSA positions in OSPF interface.
Find the OSPF interface in nexthop_calculation using
the position in the router LSA. This is possible because
the only time nexthop_calculation needs to look up interfaces
is when dealing with its own Router LSA.

This has the following advantages:
 - Multiple PtP interfaces with the same IP address between two routers.
 - Use Unnumbered PtP on just one end of the link.
 - Faster OI lookup for the OSPF interface and only
   done once for PtoP links.

*ospf_interface.h: (struct ospf_interface) Add storage for
		   storing router LSA position.

*ospf_interface.c: (ospf_if_lookup_by_lsa_pos)
		   lookup OSPF I/F in an area using LSA position.

*ospf_lsa.c: (router_lsa_link_set) record Router LSA position.

*ospf_spf.c: (ospf_spf_next) Count and pass along lsa position.
	     (ospf_nexthop_calculation) Add lsa position argument.
	     call ospf_if_lookup_by_lsa_pos() for OSFP interface handle.
	     Clean up and remove all calls ospf_if_is_configured() the
	     rest. Adjust a few debug logs.

Signed-off-by: Joakim Tjernlund <Joakim.Tjernlund@transmode.se>
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
diff --git a/ospfd/ospf_interface.h b/ospfd/ospf_interface.h
index 6db8877..9de6550 100644
--- a/ospfd/ospf_interface.h
+++ b/ospfd/ospf_interface.h
@@ -127,6 +127,10 @@
   /* OSPF Area. */
   struct ospf_area *area;
 
+  /* Position range in Router LSA */
+  uint16_t lsa_pos_beg; /* inclusive, >= */
+  uint16_t lsa_pos_end; /* exclusive, <  */
+
   /* Interface data from zebra. */
   struct interface *ifp;
   struct ospf_vl_data *vl_data;		/* Data for Virtual Link */
@@ -244,6 +248,8 @@
 
 extern int ospf_if_is_up (struct ospf_interface *);
 extern struct ospf_interface *ospf_if_exists (struct ospf_interface *);
+extern struct ospf_interface *ospf_if_lookup_by_lsa_pos (struct ospf_area *,
+							 int);
 extern struct ospf_interface *ospf_if_lookup_by_local_addr (struct ospf *,
 							    struct interface
 							    *,