bridge: Introduce br_send_bpdu_finish

The function dev_queue_xmit_skb_sk is unncessary and very confusing.
Introduce br_send_bpdu_finish to remove the need for dev_queue_xmit_skb_sk,
and have br_send_bpdu_finish call dev_queue_xmit.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: David S. Miller <davem@davemloft.net>