tipc: Introduce routine to enqueue a chain of messages on link tx queue

Create a helper routine to enqueue a chain of sk_buffs to a link's
transmit queue.  It improves readability and the new function is
anticipated to be used more than just once in the future as well.

Signed-off-by: Allan Stephens <allan.stephens@windriver.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>