clocksource: Check per-CPU clock synchronization when marked unstable

clocksource: Check per-CPU clock synchronization when marked unstable

Some sorts of per-CPU clock sources have a history of going out of
synchronization with each other.  However, this problem has purportedy
been solved in the past ten years.  Except that it is all too possible
that the problem has instead simply been made less likely, which might
mean that some of the occasional "Marking clocksource 'tsc' as unstable"
messages might be due to desynchronization.  How would anyone know?

This commit therefore adds CPU-to-CPU synchronization checking
for newly unstable clocksource that are marked with the new
CLOCK_SOURCE_VERIFY_PERCPU flag.  Lists of desynchronized CPUs are
printed, with the caveat that if it is the reporting CPU that is itself
desynchronized, it will appear that all the other clocks are wrong.
Just like in real life.

Cc: John Stultz <john.stultz@linaro.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Stephen Boyd <sboyd@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Mark Rutland <Mark.Rutland@arm.com>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Andi Kleen <ak@linux.intel.com>
Reported-by: Chris Mason <clm@fb.com>
[ paulmck: Add "static" to clocksource_verify_one_cpu() per kernel test robot feedback. ]
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>