block: fix inaccurate io_ticks

hulk inclusion
category: bugfix
bugzilla: 31388
CVE: NA
---------------------------

After introducing commit 5b18b5a7 ("block: delete part_round_stats
and switch to less precise counting"), '%util' accounted by iostat
will be over reality data. In fact, the device is quite idle, but
iostat may show '%util' as a big number (e.g. 50%). It can produce by fio:

fio --name=1 --direct=1 --bs=4k --rw=read --filename=/dev/sda \
                   --thinktime=4ms --runtime=180

We fix this by reserving part_round_stats() in io start path.

fixes: 5b18b5a7 ("block: delete part_round_stats and switch to less precise counting")
Signed-off-by: Yufen Yu <yuyufen@huawei.com>
Reviewed-by: Hou Tao <houtao1@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>