net: rtnetlink: fdb dflt dump must set idx used for cb->arg[0]

In rtnl_fdb_dump() when the fdb_dump ndo op is not populated
we never set the idx value so that cb->arg[0] is always 0.
Resulting in a endless loop of messages.

Introduced with this commit,

commit 090096bf3db1c281ddd034573260045888a68fea
Author: Vlad Yasevich <vyasevic@redhat.com>
Date:   Wed Mar 6 15:39:42 2013 +0000

    net: generic fdb support for drivers without ndo_fdb_<op>

CC: Vlad Yasevich <vyasevic@redhat.com>
Signed-off-by: John Fastabend <john.r.fastabend@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/rtnetlink.c

@@ -2262,7 +2262,7 @@ skip:
  * @dev: netdevice
  *
  * Default netdevice operation to dump the existing unicast address list.
- * Returns zero on success.
+ * Returns number of addresses from list put in skb.
  */
 int ndo_dflt_fdb_dump(struct sk_buff *skb,
 		      struct netlink_callback *cb,
@@ -2303,7 +2303,7 @@ static int rtnl_fdb_dump(struct sk_buff *skb, struct netlink_callback *cb)
 		if (dev->netdev_ops->ndo_fdb_dump)
 			idx = dev->netdev_ops->ndo_fdb_dump(skb, cb, dev, idx);
 		else
-			ndo_dflt_fdb_dump(skb, cb, dev, idx);
+			idx = ndo_dflt_fdb_dump(skb, cb, dev, idx);
 	}
 	rcu_read_unlock();