fib_trie: Correctly handle case of key == 0 in leaf_walk_rcu

In the case of a trie that had no tnodes with a key of 0 the initial
look-up would fail resulting in an out-of-bounds cindex on the first tnode.
This resulted in an entire trie being skipped.

In order resolve this I have updated the cindex logic in the initial
look-up so that if the key is zero we will always traverse the child zero
path.

Fixes: 8be33e95 ("fib_trie: Fib walk rcu should take a tnode and key instead of a trie and a leaf")
Reported-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: Alexander Duyck <alexander.h.duyck@redhat.com>
Tested-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/fib_trie.c

@@ -1530,7 +1530,7 @@ static struct key_vector *leaf_walk_rcu(struct key_vector **tn, t_key key)
 	do {
 		/* record parent and next child index */
 		pn = n;
-		cindex = get_index(key, pn);
+		cindex = key ? get_index(key, pn) : 0;
 
 		if (cindex >> pn->bits)
 			break;