prefill_possible_map() reinitializes the cpu_possible_map by setting the
possible cpu bits and clearing all other bits up to NR_CPUS.

This is technically always correct because cpu_possible_map is statically
allocated and sized NR_CPUS. With CPUMASK_OFFSTACK and DEBUG_PER_CPU_MAPS
enabled the bounds check of cpu masks happens on nr_cpu_ids. nr_cpu_ids is
initialized to NR_CPUS and only limited after the set/clear bit loops have
been executed. 

But if the system was booted with "nr_cpus=N" on the command line, where N
is < NR_CPUS then nr_cpu_ids is limited in the parameter parsing function
before prefill_possible_map() is invoked. As a consequence the cpumask
bounds check triggers when clearing the bits past nr_cpu_ids.

Add a helper which allows to reset cpu_possible_map w/o the bounds check
and then set only the possible bits which are well inside bounds.
Reported-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: 0x7f454c46@gmail.com
Cc: Jan Beulich <JBeulich@novell.com>
Link: http://lkml.kernel.org/r/alpine.DEB.2.20.1612131836050.3415@nanosSigned-off-by: Thomas Gleixner <tglx@linutronix.de>