Impact: stack usage reduction

With some care, we can avoid needing a temporary cpumask (we can't
really allocate here, since we can't fail).

This version calls cpuset_cpus_allowed_locked() with the task_rq_lock
held.  I'm fairly sure this works, but there might be a deadlock
hiding.

And of course, we can't get rid of the last cpumask on stack until we
can use cpumask_of_node instead of node_to_cpumask.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Ingo Molnar <mingo@elte.hu>