cpufreq: Prevent problems in update_policy_cpu() if last_cpu == new_cpu

If update_policy_cpu() is invoked with the existing policy->cpu itself
as the new-cpu parameter, then a lot of things can go terribly wrong.

In its present form, update_policy_cpu() always assumes that the new-cpu
is different from policy->cpu and invokes other functions to perform their
respective updates. And those functions implement the actual update like
this:

per_cpu(..., new_cpu) = per_cpu(..., last_cpu);
per_cpu(..., last_cpu) = NULL;

Thus, when new_cpu == last_cpu, the final NULL assignment makes the per-cpu
references vanish into thin air! (memory leak). From there, it leads to more
problems: cpufreq_stats_create_table() now doesn't find the per-cpu reference
and hence tries to create a new sysfs-group; but sysfs already had created
the group earlier, so it complains that it cannot create a duplicate filename.
In short, the repercussions of a rather innocuous invocation of
update_policy_cpu() can turn out to be pretty nasty.

Ideally update_policy_cpu() should handle this situation (new == last)
gracefully, and not lead to such severe problems. So fix it by adding an
appropriate check.

Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
Tested-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>