tipc: use generic SKB list APIs to manage link transmission queue

Use standard SKB list APIs associated with struct sk_buff_head to
manage link transmission queue, having relevant code more clean.

Signed-off-by: Ying Xue <ying.xue@windriver.com>
Reviewed-by: Jon Maloy <jon.maloy@ericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --cc net/tipc/bcast.c
Simple merge

diff --cc net/tipc/link.c
Simple merge

diff --cc net/tipc/link.h
Simple merge

diff --cc net/tipc/msg.c
Simple merge

diff --cc net/tipc/msg.h
Simple merge