ospf6d: fix warnings from recent prefix bit commit

* lib/prefix.h
  * prefix6_bit(): add IPv6 wrapper for prefix_bit()
* ospf6d/ospf6_lsdb.c
  * ospf6_lsdb_type_router_head(): employ prefix6_bit()
  * ospf6_lsdb_type_head(): idem
diff --git a/lib/prefix.h b/lib/prefix.h
index d370720..a7598b7 100644
--- a/lib/prefix.h
+++ b/lib/prefix.h
@@ -137,6 +137,12 @@
   return (prefix[offset] >> shift) & 1;
 }
 
+static inline unsigned int
+prefix6_bit (const struct in6_addr *prefix, const u_char prefixlen)
+{
+  return prefix_bit((const u_char *) &prefix->s6_addr, prefixlen);
+}
+
 /* Prototypes. */
 extern int afi2family (int);
 extern int family2afi (int);
diff --git a/ospf6d/ospf6_lsdb.c b/ospf6d/ospf6_lsdb.c
index d2e3e04..280bdf9 100644
--- a/ospf6d/ospf6_lsdb.c
+++ b/ospf6d/ospf6_lsdb.c
@@ -381,7 +381,7 @@
   /* Walk down tree. */
   while (node && node->p.prefixlen <= key.prefixlen &&
 	 prefix_match (&node->p, (struct prefix *) &key))
-    node = node->link[prefix_bit(&key.prefix, node->p.prefixlen)];
+    node = node->link[prefix6_bit(&key.prefix, node->p.prefixlen)];
 
   if (node)
     route_lock_node (node);
@@ -435,7 +435,7 @@
   node = lsdb->table->top;
   while (node && node->p.prefixlen <= key.prefixlen &&
 	 prefix_match (&node->p, (struct prefix *) &key))
-    node = node->link[prefix_bit(&key.prefix, node->p.prefixlen)];
+    node = node->link[prefix6_bit(&key.prefix, node->p.prefixlen)];
 
   if (node)
     route_lock_node (node);