blk-cgroup: use group allocation/free of per-cpu counters API

Use group allocation/free of per-cpu counters api to accelerate
blkg_rwstat_init/exit() and simplify code.

Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Link: https://lore.kernel.org/r/20240325035955.50019-1-wangkefeng.wang@huawei.com


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