tipc: Optimize creation of FIN messages

Speeds up the creation of the FIN message that terminates a TIPC
connection. The typical peer termination message is now created by
duplicating the terminating port's standard payload message header
and adjusting the message size, importance, and error code fields,
rather than building all fields of the message from scratch. A FIN
message that is directed to the port itself is created the same way.
but also requires swapping the origin and destination address fields.

In addition to reducing the work required to create FIN messages,
these changes eliminate several instances of duplicated code,

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