blk-iocost: Remove vrate member in struct ioc_now

If we trace vtime_base_rate instead of vtime_rate, there is nowhere
which accesses now->vrate except function ioc_now using now->vrate locally.
Just remove it.
Signed-off-by: Kemeng Shi <shikemeng@huawei.com>
Acked-by: Tejun Heo <tj@kernel.org>
Link: https://lore.kernel.org/r/20221018121932.10792-5-shikemeng@huawei.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-iocost.c

@@ -556,7 +556,6 @@ struct ioc_now {
 	u64				now_ns;
 	u64				now;
 	u64				vnow;
-	u64				vrate;
 };
 
 struct iocg_wait {
@@ -1020,10 +1019,11 @@ static void ioc_adjust_base_vrate(struct ioc *ioc, u32 rq_wait_pct,
 static void ioc_now(struct ioc *ioc, struct ioc_now *now)
 {
 	unsigned seq;
+	u64 vrate;
 
 	now->now_ns = ktime_get();
 	now->now = ktime_to_us(now->now_ns);
-	now->vrate = atomic64_read(&ioc->vtime_rate);
+	vrate = atomic64_read(&ioc->vtime_rate);
 
 	/*
 	 * The current vtime is
@@ -1036,7 +1036,7 @@ static void ioc_now(struct ioc *ioc, struct ioc_now *now)
 	do {
 		seq = read_seqcount_begin(&ioc->period_seqcount);
 		now->vnow = ioc->period_at_vtime +
-			(now->now - ioc->period_at) * now->vrate;
+			(now->now - ioc->period_at) * vrate;
 	} while (read_seqcount_retry(&ioc->period_seqcount, seq));
 }