tipc: convert tipc_bearers array to pointer list

As part of the effort to introduce RCU protection for the bearer
list, we first need to change it to a list of pointers.

Signed-off-by: Ying Xue <ying.xue@windriver.com>
Reviewed-by: Erik Hugne <erik.hugne@ericsson.com>
Reviewed-by: Jon Maloy <jon.maloy@ericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>