[CPUFREQ] fix cpumask memory leak in acpi-cpufreq on cpu hotplug.

I came across a memory leak during a cyclic cpu-online-offline test.

Signed-off-by: Yu Luming <luming.yu@intel.com>
Cc: Len Brown <lenb@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Dave Jones <davej@redhat.com>