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

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>