fix #25369772

In blk-mq device, we observed a issue that though iops is low, but iostat
shows a very high svctm & util value, which is counter-intuitive.

The root cause is that blk_account_io_start() calls part_round_stats()
before "rq->part = part" statement, so part_round_stats() will count
an inflight request to the whole device, but not for the specific
partition, then it'll update whole device's io_ticks and time_in_queue
with a stale part->stamp.

To fix this issue, if a request's part is NULL, we just don't count
it as an inflight request to the whole device.
Signed-off-by: NXiaoguang Wang <xiaoguang.wang@linux.alibaba.com>
Reviewed-by: NJoseph Qi <joseph.qi@linux.alibaba.com>