sched/fair: prevent possible infinite loop in compute_energy()

There is a race between hotplug and energy_diff which might result
in endless loop in compute_energy. When this happens, the end
condition cannot be detected.

We can store how many CPUs we need to visit at the beginning, and
bail out of the energy calculation if we visit more cpus than expected.

Change-Id: I916205877d49021bb97aca6c90a192a82d122661
Signed-off-by: Chris Redpath <chris.redpath@arm.com>
Git-commit: 8a174b4749d384865b60690cb33cf9f2dc490986
Git-repo: https://android.googlesource.com/kernel/common/


Signed-off-by: Puja Gupta <pujag@codeaurora.org>