ipv4: Fix fib_trie rebalancing, part 3

Alas current delaying of freeing old tnodes by RCU in trie_rebalance
is still not enough because we can free a top tnode before updating a
t->trie pointer.
Reported-by: Pawel Staszewski <pstaszewski@itcare.pl>
Tested-by: Pawel Staszewski <pstaszewski@itcare.pl>
Signed-off-by: Jarek Poplawski <jarkao2@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/fib_trie.c

@@ -1021,6 +1021,9 @@ static void trie_rebalance(struct trie *t, struct tnode *tn)
 				      (struct node *)tn, wasfull);
 
 		tp = node_parent((struct node *) tn);
+		if (!tp)
+			rcu_assign_pointer(t->trie, (struct node *)tn);
+
 		tnode_free_flush();
 		if (!tp)
 			break;