cpumask: set nr_cpumask_bits to nr_cpu_ids on PREEMPT_RT

As we have CPUMASK_OFFSTACK disabled on RT, we end up having the cpumap
and cpumask set with the full NR_CPUS size, 5120 bits. That causes
/sys/devices/system/node/nodeX/cpumap to display a huge array of values
instead of the cpumap containing the amount of CPUs supported by the system.

Example:

# cat /sys/devices/system/node/node0/cpumap
00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,0000000f

With CPUMASK_OFFSTACK enabled we would have:

# cat /sys/devices/system/node/node0/cpumap
f

That difference confuses libvirt, that fails to get the CPU info. Maybe other
applications are affected by that too.

The proposed change gets the desired behavior and decreases the size of the
cpumask considerably for smaller systems.

Signed-off-by: Luis Claudio R. Goncalves <lgoncalv@redhat.com>
Signed-off-by: Clark Williams <williams@redhat.com>

diff --git a/include/linux/cpumask.h b/include/linux/cpumask.h
index d08e4d2a..c7c2ec7 100644
--- a/include/linux/cpumask.h
+++ b/include/linux/cpumask.h
@@ -28,7 +28,7 @@ typedef struct cpumask { DECLARE_BITMAP(bits, NR_CPUS); } cpumask_t;
 extern int nr_cpu_ids;
 #endif

-#ifdef CONFIG_CPUMASK_OFFSTACK
+#if defined(CONFIG_CPUMASK_OFFSTACK) || defined(CONFIG_PREEMPT_RT_BASE)
 /* Assuming NR_CPUS is huge, a runtime limit is more efficient.  Also,
  * not all bits may be allocated. */
 #define nr_cpumask_bits        nr_cpu_ids