[ospf6d] Fix regression in monotonic time patch - LSA max-aging broke

* ospf6_lsa.c: (ospf6_lsa_premature_aging) set age to MAX_AGE - don't
  rely on 0 magically meaning same.
  (ospf6_lsa_age_current) handle MAXAGE.
diff --git a/ospf6d/ospf6_lsa.c b/ospf6d/ospf6_lsa.c
index 108c025..34040b8 100644
--- a/ospf6d/ospf6_lsa.c
+++ b/ospf6d/ospf6_lsa.c
@@ -207,6 +207,12 @@
     zlog_warn ("LSA: quagga_gettime failed, may fail LSA AGEs: %s",
                safe_strerror (errno));
 
+  if (lsa->header->age >= htons (MAXAGE))
+    {
+      /* LSA may have been prematurely aged */
+      lsa->header->age = htons (MAXAGE);
+      return MAXAGE;
+    }
   /* calculate age */
   ulage = now.tv_sec - lsa->birth.tv_sec;
 
@@ -239,7 +245,12 @@
   THREAD_OFF (lsa->expire);
   THREAD_OFF (lsa->refresh);
 
+  /* 
+   * The below technique to age out LSA does not work when using relative time 
+   *
   memset (&lsa->birth, 0, sizeof (struct timeval));
+   */
+  lsa->header->age = htons (MAXAGE);
   thread_execute (master, ospf6_lsa_expire, lsa, 0);
 }