ANDROID: sched/fair: unify spare capacity calculation

Given that we have a few sites where the spare capacity of a CPU is
calculated as the difference between the original capacity of the CPU
and its computed new utilization, let's unify the calculation and use
that value tracked with a local spare_cap variable.

Change-Id: I78daece7543f78d4f74edbee5e9ceb62908af507
Signed-off-by: Ionela Voinescu <ionela.voinescu@arm.com>
Git-commit: 5383285d
Git-repo: http://android.googlesource.com/kernel/common


[adharmap@codeaurora.org: spare_cap is already used in 4.14
baseline, rename that to spare_wake_cap to avoid name collision]
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>