sched/vtime: Bring up complete kcpustat accessor

Many callsites want to fetch the values of system, user, user_nice, guest
or guest_nice kcpustat fields altogether or at least a pair of these.

In that case calling kcpustat_field() for each requested field brings
unecessary overhead when we could fetch all of them in a row.

So provide kcpustat_cpu_fetch() that fetches the whole kcpustat array
in a vtime safe way under the same RCU and seqcount block.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Wanpeng Li <wanpengli@tencent.com>
Cc: Yauheni Kaliuta <yauheni.kaliuta@redhat.com>
Link: https://lkml.kernel.org/r/20191121024430.19938-3-frederic@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

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