tipc: define new functions to operate bc_lock

As we are going to do more jobs when bc_lock is released, the two
operations of holding/releasing the lock should be encapsulated with
functions. In addition, we move bc_lock spin lock into tipc_bclink
structure avoiding to define the global variable.

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>