cpumask: use cpumask_var_t in acpi-cpufreq.c

Impact: cleanup, reduce stack usage, use new cpumask API.

Replace the cpumask_t in struct drv_cmd with a cpumask_var_t.  Remove unneeded
online_policy_cpus cpumask_t in acpi_cpufreq_target.  Update refs to use
new cpumask API.

Signed-off-by: Mike Travis <travis@sgi.com>
Acked-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Ingo Molnar <mingo@elte.hu>