cgroup/rstat: Record the cumulative per-cpu time of cgroup and its descendants

The member variable bstat of the structure cgroup_rstat_cpu
records the per-cpu time of the cgroup itself, but does not
include the per-cpu time of its descendants. The per-cpu time
including descendants is very useful for calculating the
per-cpu usage of cgroups.

Although we can indirectly obtain the total per-cpu time
of the cgroup and its descendants by accumulating the per-cpu
bstat of each descendant of the cgroup. But after a child cgroup
is removed, we will lose its bstat information. This will cause
the cumulative value to be non-monotonic, thus affecting
the accuracy of cgroup per-cpu usage.

So we add the subtree_bstat variable to record the total
per-cpu time of this cgroup and its descendants, which is
similar to "cpuacct.usage*" in cgroup v1. And this is
also helpful for the migration from cgroup v1 to cgroup v2.
After adding this variable, we can obtain the per-cpu time of
cgroup and its descendants in user mode through eBPF/drgn, etc.
And we are still trying to determine how to expose it in the
cgroupfs interface.

Suggested-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Hao Jia <jiahao.os@bytedance.com>
Signed-off-by: Tejun Heo <tj@kernel.org>