blk-cgroup: fix list corruption from resetting io stat

In the Linux kernel, the following vulnerability has been resolved: blk-cgroup: fix list corruption from resetting io stat Since commit 3b8cc6298724 ("blk-cgroup: Optimize blkcg_rstat_flush()"), each iostat instance is added to blkcg percpu list, so blkcg_reset_stats() can't reset the stat instance by memset(), otherwise the llist may be corrupted. Fix the issue by only resetting the counter part.