net: suppress lockdep-RCU false positive in FIB trie.

Allow fib_find_node() to be called either under rcu_read_lock()
protection or with RTNL held.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: NEric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/fib_trie.c

@@ -961,7 +961,9 @@ fib_find_node(struct trie *t, u32 key)
 	struct node *n;
 
 	pos = 0;
-	n = rcu_dereference(t->trie);
+	n = rcu_dereference_check(t->trie,
+				  rcu_read_lock_held() ||
+				  lockdep_rtnl_is_held());
 
 	while (n != NULL &&  NODE_TYPE(n) == T_TNODE) {
 		tn = (struct tnode *) n;