psi: rename psi fields in preparation for psi trigger addition

Renaming psi_group structure member fields used for calculating psi totals
and averages for clear distinction between them and trigger-related fields
that will be added next.

Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Change-Id: I1d0cc0fb511f6df688e73606e4bcc5f08f54c92e
Patch-mainline: linux-mm @ 03/19/2019, 23:56
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>