When I was looking at the cpuidle code after the Sudeeps's problem report,
it occured to me that we had some pointless overhead there, so two
changes to reduce it follow.
[1/2] Make the fallback to to default_idle_call() in call_cpuidle()
unnecessary and drop it.
[2/2] Make menu_select() avoid checking states that don't need to
(or even shouldn't) be checked when making the selection.