tipc: Add missing broadcast link lock when sending NACK

Ensures that any attempt to send a NACK message over TIPC's broadcast
link has exclusive access to the link's main data structures, to prevent
interference with a simultaneous attempt to send other broadcast link
traffic (such as application-generated multicast messages).
Signed-off-by: Allan Stephens <allan.stephens@windriver.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>

net/tipc/bcast.c

@@ -340,8 +340,10 @@ static void bclink_send_nack(struct tipc_node *n_ptr)
 		msg_set_bcgap_to(msg, n_ptr->bclink.gap_to);
 		msg_set_bcast_tag(msg, tipc_own_tag);
 
+		spin_lock_bh(&bc_lock);
 		tipc_bearer_send(&bcbearer->bearer, buf, NULL);
 		bcl->stats.sent_nacks++;
+		spin_unlock_bh(&bc_lock);
 		buf_discard(buf);
 
 		/*