rackmeter: Use vtime aware kcpustat accessor

Now that we have a vtime safe kcpustat accessor, use it to fetch
CPUTIME_NICE and fix frozen kcpustat values on nohz_full CPUs.

Reported-by: Yauheni Kaliuta <yauheni.kaliuta@redhat.com>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Wanpeng Li <wanpengli@tencent.com>
Link: https://lkml.kernel.org/r/20191121024430.19938-7-frederic@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>