If we're stopping a secondary cpu, don't let sched_choosecpu() short-circuit

If we're stopping a secondary cpu, don't let sched_choosecpu() short-circuit
and return the current current CPU, otherwise sched_stop_secondary_cpus()
will spin forever trying to empty its run queues.  Fixes hangs during suspend
that many people reported over the last couple of days.

ok bcook@, guenther@