blk-iocost: make ioc_now->now and ioc->period_at 64bit

They are in microseconds and wrap in around 1.2 hours with u32. While
unlikely, confusions from wraparounds are still possible. We aren't saving
anything meaningful by keeping these u32. Let's make them u64.
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-iocost.c

@@ -409,7 +409,7 @@ struct ioc {
 	atomic64_t			vtime_rate;
 
 	seqcount_spinlock_t		period_seqcount;
-	u32				period_at;	/* wallclock starttime */
+	u64				period_at;	/* wallclock starttime */
 	u64				period_at_vtime; /* vtime starttime */
 
 	atomic64_t			cur_period;	/* inc'd each period */
@@ -508,7 +508,7 @@ struct ioc_cgrp {
 
 struct ioc_now {
 	u64				now_ns;
-	u32				now;
+	u64				now;
 	u64				vnow;
 	u64				vrate;
 };