[IPV4]: Thresholds in fib_trie.c are used as consts, so make them const.

There are several thresholds for trie fib hash management. They are used
in the code as a constants. Make them constants from the compiler point of
view.
Signed-off-by: Denis V. Lunev <den@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/fib_trie.c

@@ -299,10 +299,10 @@ static inline void check_tnode(const struct tnode *tn)
 	WARN_ON(tn && tn->pos+tn->bits > 32);
 }
 
-static int halve_threshold = 25;
-static int inflate_threshold = 50;
-static int halve_threshold_root = 8;
-static int inflate_threshold_root = 15;
+static const int halve_threshold = 25;
+static const int inflate_threshold = 50;
+static const int halve_threshold_root = 8;
+static const int inflate_threshold_root = 15;
 
 
 static void __alias_free_mem(struct rcu_head *head)