tipc: fix bug in broadcast synch message create function

In commit d999297c
("tipc: reduce locking scope during packet reception") we introduced
a new function tipc_build_bcast_sync_msg(), which carries initial
synchronization data between two nodes at first contact and at
re-contact. In this function, we missed to add synchronization data,
with the effect that the broadcast link endpoints will fail to
synchronize correctly at re-contact between a running and a restarted
node. All other cases work as intended.

With this commit, we fix this bug.

Signed-off-by: Jon Maloy <jon.maloy@ericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>