fib_trie: only calc for the un-first node

This is a enhancement.

for the first node in fib_trie, newpos is 0, bit is 1.
Only for the leaf or node with unmatched key need calc pos.
Signed-off-by: baker.zhang <baker.kernel@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/fib_trie.c

@@ -1117,12 +1117,8 @@ static struct list_head *fib_insert_node(struct trie *t, u32 key, int plen)
 		 *  first tnode need some special handling
 		 */
 
-		if (tp)
-			pos = tp->pos+tp->bits;
-		else
-			pos = 0;
-
 		if (n) {
+			pos = tp ? tp->pos+tp->bits : 0;
 			newpos = tkey_mismatch(key, pos, n->key);
 			tn = tnode_new(n->key, newpos, 1);
 		} else {