sched: reduce stack size requirements in kernel/sched.c

Impact: cleanup

  * use node_to_cpumask_ptr in place of node_to_cpumask to reduce stack
    requirements in sched.c

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