blk-throttle: support prioritized processing of metadata

Currently, blk-throttle handle all IO fifo, hence if data IO is
throttled and then meta IO is dispatched, the meta IO will have to wait
for the data IO, causing priority inversion problems.

This patch support to handle metadata first and then pay debt while
throttling data.

Test script: use cgroup v1 to throttle root cgroup, then create new
dir and file while write back is throttled

test() {
  mkdir /mnt/test/xxx
  touch /mnt/test/xxx/1
  sync /mnt/test/xxx
  sync /mnt/test/xxx
}

mkfs.ext4 -F /dev/nvme0n1 -E lazy_itable_init=0,lazy_journal_init=0
mount /dev/nvme0n1 /mnt/test

echo "259:0 $((1024*1024))" > /sys/fs/cgroup/blkio/blkio.throttle.write_bps_device
dd if=/dev/zero of=/mnt/test/foo1 bs=16M count=1 conv=fdatasync status=none &
sleep 4

time test
echo "259:0 0" > /sys/fs/cgroup/blkio/blkio.throttle.write_bps_device

sleep 1
umount /dev/nvme0n1

Test result: time cost for creating new dir and file
before this patch:  14s
after this patch:   0.1s

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Acked-by: Tejun Heo <tj@kernel.org>
Link: https://lore.kernel.org/r/20240903135149.271857-3-yukuai1@huaweicloud.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>