Use generic io stats accounting help functions (generic_{start,end}_io_acct)
to simplify io stat accounting.
Signed-off-by: NGu Zheng <guz.fnst@cn.fujitsu.com>
Acked-by: NKent Overstreet <kmo@datera.io>
Signed-off-by: NJens Axboe <axboe@fb.com>