cpumask: use work_on_cpu in acpi/cstate.c

Impact: use new cpumask API to reduce stack usage

Replace the saving of current->cpus_allowed and set_cpus_allowed_ptr() with
a work_on_cpu function for the acpi_processor_ffh_cstate_probe() function.

Basically splits acpi_processor_ffh_cstate_probe() into two functions, the
other being acpi_processor_ffh_cstate_probe_cpu which is the work function
run on the designated cpu.

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