rcu: Fix a sparse warning in rcu_initiate_boost()

This commit annotates rcu_initiate_boost() fixes the following sparse
warning:

	kernel/rcu/tree_plugin.h:1494:13: warning: context imbalance in 'rcu_initiate_boost' - unexpected unlock
Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>

kernel/rcu/tree_plugin.h

@@ -1259,6 +1259,7 @@ static int rcu_boost_kthread(void *arg)
  * about it going away.
  */
 static void rcu_initiate_boost(struct rcu_node *rnp, unsigned long flags)
+	__releases(rnp->lock)
 {
 	struct task_struct *t;
 
@@ -1494,6 +1495,7 @@ static void rcu_prepare_kthreads(int cpu)
 #else /* #ifdef CONFIG_RCU_BOOST */
 
 static void rcu_initiate_boost(struct rcu_node *rnp, unsigned long flags)
+	__releases(rnp->lock)
 {
 	raw_spin_unlock_irqrestore(&rnp->lock, flags);
 }