tipc: fix imbalance read_unlock_bh in __tipc_nl_add_monitor()

In the error handling case of nla_nest_start() failed read_unlock_bh()
is called  to unlock a lock that had not been taken yet. sparse warns
about the context imbalance as the following:

net/tipc/monitor.c:799:23: warning:
 context imbalance in '__tipc_nl_add_monitor' - different lock contexts for basic block

Fixes: cf6f7e1d ('tipc: dump monitor attributes')
Signed-off-by: Wei Yongjun <weiyj.lk@gmail.com>
Acked-by: Ying Xue <ying.xue@windriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/tipc/monitor.c

@@ -794,10 +794,10 @@ int __tipc_nl_add_monitor(struct net *net, struct tipc_nl_msg *msg,
 	return 0;
 
 attr_msg_full:
+	read_unlock_bh(&mon->lock);
 	nla_nest_cancel(msg->skb, attrs);
 msg_full:
 	genlmsg_cancel(msg->skb, hdr);
-	read_unlock_bh(&mon->lock);
 
 	return -EMSGSIZE;
 }