sched/cpuacct: Split usage accounting into user_usage and sys_usage

Sometimes, cpuacct.usage is not detailed enough to see how much CPU
usage a group had. We want to know how much time it used in user mode
and how much in kernel mode.

This patch introduces more files to give this information:

 # ls /sys/fs/cgroup/cpuacct/cpuacct.usage*
 /sys/fs/cgroup/cpuacct/cpuacct.usage
 /sys/fs/cgroup/cpuacct/cpuacct.usage_percpu
 /sys/fs/cgroup/cpuacct/cpuacct.usage_user
 /sys/fs/cgroup/cpuacct/cpuacct.usage_percpu_user
 /sys/fs/cgroup/cpuacct/cpuacct.usage_sys
 /sys/fs/cgroup/cpuacct/cpuacct.usage_percpu_sys

... while keeping the ABI with the existing counter.

Signed-off-by: Dongsheng Yang <yangds.fnst@cn.fujitsu.com>
[ Ported to newer kernels. ]
Signed-off-by: Zhao Lei <zhaolei@cn.fujitsu.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Tejun Heo <tj@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Tejun Heo <htejun@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/aa171da036b520b51c79549e9b3215d29473f19d.1458635566.git.zhaolei@cn.fujitsu.com


Signed-off-by: Ingo Molnar <mingo@kernel.org>