clocksource: Limit number of CPUs checked for clock synchronization

clocksource: Limit number of CPUs checked for clock synchronization

Currently, if skew is detected on a clock marked CLOCK_SOURCE_VERIFY_PERCPU,
that clock is checked on all CPUs.  This is thorough, but might not be
what you want on a system with a few tens of CPUs, let alone a few hundred
of them.

Therefore, by default check only up to eight randomly chosen CPUs.
Also provide a new clocksource.verify_n_cpus kernel boot parameter.
A value of -1 says to check all of the CPUs, and a non-negative value says
to randomly select that number of CPUs, without concern about selecting
the same CPU multiple times.  However, make use of a cpumask so that a
given CPU will be checked at most once.

Link: https://lore.kernel.org/lkml/202104291438.PuHsxRkl-lkp@intel.com/
Link: https://lore.kernel.org/lkml/20210429140440.GT975577@paulmck-ThinkPad-P17-Gen-1
Link: https://lore.kernel.org/lkml/20210425224540.GA1312438@paulmck-ThinkPad-P17-Gen-1/
Link: https://lore.kernel.org/lkml/20210420064934.GE31773@xsang-OptiPlex-9020/
Link: https://lore.kernel.org/lkml/20210106004013.GA11179@paulmck-ThinkPad-P72/
Link: https://lore.kernel.org/lkml/20210414043435.GA2812539@paulmck-ThinkPad-P17-Gen-1/
Link: https://lore.kernel.org/lkml/20210419045155.GA596058@paulmck-ThinkPad-P17-Gen-1/
Suggested-by: Thomas Gleixner <tglx@linutronix.de> # For verify_n_cpus=1.
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>
Cc: Feng Tang <feng.tang@intel.com>
Cc: Xing Zhengjun <zhengjun.xing@linux.intel.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>