cpumask: Add alloc_cpumask_var_node()

Impact: New API

This will be needed in x86 code to allocate the domain and old_domain
cpumasks on the same node as where the containing irq_cfg struct is
allocated.

(Also fixes double-dump_stack on rare CONFIG_DEBUG_PER_CPU_MAPS case)

Signed-off-by: Mike Travis <travis@sgi.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au> (re-impl alloc_cpumask_var)