x86/Kconfig: Enforce limit of 512 CPUs with MAXSMP and no CPUMASK_OFFSTACK

The help text of NR_CPUS says that the maximum number of CPUs supported
without CPUMASK_OFFSTACK is 512. However, NR_CPUS_RANGE_END allows this
limit to be bypassed by MAXSMP even if CPUMASK_OFFSTACK is not set.

This scenario can currently only happen in the RT tree, since it has
"select CPUMASK_OFFSTACK if !PREEMPT_RT_FULL" in MAXSMP. However,
even if we ignore the RT tree, checking for MAXSMP in addition to
CPUMASK_OFFSTACK is redundant.

Signed-off-by: Scott Wood <swood@redhat.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Mike Travis <mike.travis@hpe.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: x86-ml <x86@kernel.org>
Link: https://lkml.kernel.org/r/20191012070054.28657-1-swood@redhat.com