cfq-iosched: Fix a memory leak of per cpu stats for root group

We allocated per cpu stats struct for root group but did not free it.
Fix it.
Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>

block/cfq-iosched.c

@@ -3940,6 +3940,11 @@ static void cfq_exit_queue(struct elevator_queue *e)
 	 */
 	if (wait)
 		synchronize_rcu();
+
+#ifdef CONFIG_CFQ_GROUP_IOSCHED
+	/* Free up per cpu stats for root group */
+	free_percpu(cfqd->root_group.blkg.stats_cpu);
+#endif
 	kfree(cfqd);
 }