ipv6: Fix __ip6_route_redirect

In my last commit (a3c00e46: ipv6: Remove BACKTRACK macro), the changes in
__ip6_route_redirect is incorrect.  The following case is missed:
1. The for loop tries to find a valid gateway rt. If it fails to find
   one, rt will be NULL.
2. When rt is NULL, it is set to the ip6_null_entry.
3. The newly added 'else if', from a3c00e46, will stop the backtrack from
   happening.
Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/route.c

@@ -1242,12 +1242,16 @@ static struct rt6_info *__ip6_route_redirect(struct net *net,
 		rt = net->ipv6.ip6_null_entry;
 	else if (rt->dst.error) {
 		rt = net->ipv6.ip6_null_entry;
-	} else if (rt == net->ipv6.ip6_null_entry) {
+		goto out;
+	}
+
+	if (rt == net->ipv6.ip6_null_entry) {
 		fn = fib6_backtrack(fn, &fl6->saddr);
 		if (fn)
 			goto restart;
 	}
 
+out:
 	dst_hold(&rt->dst);
 
 	read_unlock_bh(&table->tb6_lock);