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.

Suggested-by: Thomas Gleixner <[email protected]> # For verify_n_cpus=1.
Signed-off-by: Paul E. McKenney <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Acked-by: Feng Tang <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

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