[CPUFREQ] cpumask: don't put a cpumask on the stack in x86...cpufreq/powernow-k8.c

It's still mugging the current process's cpumask, but as comment in
1ff6e97f

 says, it's not a trivial fix.

So, at least we can use a cpumask_var_t to do the Wrong Thing the Right Way :)

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
To: cpufreq@vger.kernel.org
Cc: Mark Langsdorf <mark.langsdorf@amd.com>
Signed-off-by: Dave Jones <davej@redhat.com>