to #29139300

commit 5f0ed774ed2914decfd397569fface997532e94d upstream

This isn't exactly the same as the previous count, as it includes
requests for all devices. But that really doesn't matter, if we have
more than the threshold (16) queued up, flush it. It's not worth it
to have an expensive list loop for this.

[Hongnan Li] performance evaluation
Performance results running fio(ioengine=io_uring,iodepth=256)

bs      IOPS(randread nomerges=0)   IOPS(randread nomerges=2)
           before / after                before / after
-----  ---------------------------  ---------------------------
512         818K / 840K                   855K / 897K
1k          816K / 842K                   853K / 898K
2k          820K / 839K                   850K / 899K
4k          818K / 840K                   852K / 895K
8k          574K / 574K                   574K / 574K
Reviewed-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NJens Axboe <axboe@kernel.dk>
Signed-off-by: NHongnan Li <hongnan.li@linux.alibaba.com>
Reviewed-by: NJoseph Qi <joseph.qi@linux.alibaba.com>
Signed-off-by: NJoseph Qi <joseph.qi@linux.alibaba.com>