rcuclassic: fix compilation NG

fix:

  CC      kernel/rcuclassic.o
kernel/rcuclassic.c: In function '__rcu_process_callbacks':
kernel/rcuclassic.c:561: error: 'flags' undeclared (first use in this function)
kernel/rcuclassic.c:561: error: (Each undeclared identifier is reported only once
kernel/rcuclassic.c:561: error: for each function it appears in.)

Declare missing variable flags.

Signed-off-by: Hiroshi Shimamoto <h-shimamoto@ct.jp.nec.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/rcuclassic.c

@@ -557,6 +557,8 @@ static void __rcu_process_callbacks(struct rcu_ctrlblk *rcp,
 		local_irq_enable();
 
 		if (rcu_batch_after(rdp->batch, rcp->pending)) {
+			unsigned long flags;
+
 			/* and start it/schedule start if it's a new batch */
 			spin_lock_irqsave(&rcp->lock, flags);
 			if (rcu_batch_after(rdp->batch, rcp->pending)) {