sched/cpuacct: Allocate per_cpu cpuusage for root cpuacct statically

This is a preparation, so later we can initialize cpuacct
earlier.

Signed-off-by: Li Zefan <lizefan@huawei.com>
Cc: Tejun Heo <tj@kernel.org>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/51553822.5000403@huawei.com


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