psi: split update_stats into parts

Split update_stats into collect_percpu_times and update_averages for
collect_percpu_times to be reused later inside psi monitor.

Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Change-Id: I3b9b571bd8bb9264f077a69ba6cabf23c36a4fb7
Patch-mainline: linux-mm @ 03/19/2019, 23:56
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>