From: Rusty Russell <rusty@rustcorp.com.au>,
      Nick Piggin <piggin@cyberone.com.au>

The current sched_balance_exec() sets the task's cpus_allowed mask
temporarily to move it to a different CPU.  This has several issues,
including the fact that a task will see its affinity at a bogus value.

So we change the migration_req_t to explicitly specify a destination CPU,
rather than the migration thread deriving it from cpus_allowed.  If the
requested CPU is no longer valid (racing with another set_cpus_allowed,
say), it can be ignored: if the task is not allowed on this CPU, there will
be another migration request pending.

This change allows sched_balance_exec() to tell the migration thread what
to do without changing the cpus_allowed mask.

So we rename __set_cpus_allowed() to move_task(), as the cpus_allowed mask
is now set by the caller.  And move_task_away(), which the migration thread
uses to actually perform the move, is renamed __move_task().

I also ignore offline CPUs in sched_best_cpu(), so sched_migrate_task()
doesn't need to check for offline CPUs.

Ulterior motive: this approach also plays well with CPU Hotplug.
Previously that patch might have seen a task with cpus_allowed only
containing the dying CPU (temporarily due to sched_balance_exec) and
forcibly reset it to all cpus, which might be wrong.  The other approach is
to hold the cpucontrol sem around sched_balance_exec(), which is too much
of a bottleneck.