x86: tsc: Rework time_cpufreq_notifier()

There are problems with running time_cpufreq_notifier() on SMP
systems.

First off, the rdtsc() called from there runs on the CPU executing
that code and not necessarily on the CPU whose sched_clock() rate is
updated which is questionable at best.

Second, in the cases when the frequencies of all CPUs in an SMP
system are always in sync, it is not sufficient to update just
one of them or the set associated with a given cpufreq policy on
frequency changes - all CPUs in the system should be updated and
that would require more than a simple transition notifier.

Note, however, that the underlying issue (the TSC rate depending on
the CPU frequency) has not been present in hardware shipping for the
last few years and in quite a few relevant cases (acpi-cpufreq in
particular) running time_cpufreq_notifier() will cause the TSC to
be marked as unstable anyway.

For this reason, make time_cpufreq_notifier() simply mark the TSC
as unstable and give up when run on SMP and only try to carry out
any adjustments otherwise.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>

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