tipc: Optimize message initialization routine

This patch eliminates the rarely-used "error code" argument
when initializing a TIPC message header, since the default
value of zero is the desired result in most cases; the few
exceptional cases now set the error code explicitly.

Signed-off-by: Allan Stephens <allan.stephens@windriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>