ifb: fix oops when loading the ifb failed

If __rtnl_link_register() return faild when loading the ifb, it will
take the wrong path and get oops, so fix it just like dummy.

Signed-off-by: Ding Tianhong <dingtianhong@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>