ipv4: Use const'ify fib_result deep in the route call chains.

The only troublesome bit here is __mkroute_output which wants
to override res->fi and res->type, compute those in local
variables instead.

Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/net/ip_fib.h b/include/net/ip_fib.h
index b3019d8..523a170 100644
--- a/include/net/ip_fib.h
+++ b/include/net/ip_fib.h
@@ -202,7 +202,7 @@
 extern void __net_exit fib4_rules_exit(struct net *net);
 
 #ifdef CONFIG_IP_ROUTE_CLASSID
-extern u32 fib_rules_tclass(struct fib_result *res);
+extern u32 fib_rules_tclass(const struct fib_result *res);
 #endif
 
 extern int fib_lookup(struct net *n, struct flowi *flp, struct fib_result *res);