Impact: stack usage reduction

Dynamically allocating cpumasks (when CONFIG_CPUMASK_OFFSTACK) saves
stack space.  cpumask_var_t is just a struct cpumask for
!CONFIG_CPUMASK_OFFSTACK.

In this case, we always alloced, but we don't need to any more.
Signed-off-by: NRusty Russell <rusty@rustcorp.com.au>
Signed-off-by: NIngo Molnar <mingo@elte.hu>