cpumask: replace CPUMASK_ALLOC etc with cpumask_var_t

Impact: cleanup

There's only one user, and it's a fairly easy conversion.

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