clocksource: Do pairwise clock-desynchronization checking

clocksource: Do pairwise clock-desynchronization checking

Although smp_call_function() has the advantage of simplicity, using
it to check for cross-CPU clock desynchronization means that any CPU
being slow reduces the sensitivity of the checking across all CPUs.
And it is not uncommon for smp_call_function() latencies to be in the
hundreds of microseconds.

This commit therefore switches to smp_call_function_single(), so that
delays from a given CPU affect only those measurements involving that
particular CPU.

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>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>