ipv6_route_seq_next should increase position index

if seq_file .next fuction does not change position index,
read after some lseek can generate unexpected output.

https://bugzilla.kernel.org/show_bug.cgi?id=206283
Signed-off-by: Vasily Averin <vvs@virtuozzo.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/ip6_fib.c

@@ -2495,23 +2495,20 @@ static void *ipv6_route_seq_next(struct seq_file *seq, void *v, loff_t *pos)
 	struct net *net = seq_file_net(seq);
 	struct ipv6_route_iter *iter = seq->private;
 
+	++(*pos);
 	if (!v)
 		goto iter_table;
 
 	n = rcu_dereference_bh(((struct fib6_info *)v)->fib6_next);
-	if (n) {
-		++*pos;
+	if (n)
 		return n;
-	}
 
 iter_table:
 	ipv6_route_check_sernum(iter);
 	spin_lock_bh(&iter->tbl->tb6_lock);
 	r = fib6_walk_continue(&iter->w);
 	spin_unlock_bh(&iter->tbl->tb6_lock);
 	if (r > 0) {
-		if (v)
-			++*pos;
 		return iter->w.leaf;
 	} else if (r < 0) {
 		fib6_walker_unlink(net, &iter->w);