tipc: Set unused probe field of link protocol messages to defined value

Ensures that a link reset or activate message has a "probe" field
of zero. (This field is currently unused in these messages, but this
could potentially change in future versions of TIPC.)

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