cpumask: zero extra bits in alloc_cpumask_var_node

Impact: extra safety checks during transition

When CONFIG_CPUMASKS_OFFSTACK is set, the new cpumask_ operators only
use bits up to nr_cpu_ids, not NR_CPUS.  Using the old cpus_ operators
on these masks can mean accessing undefined bits.

After some discussion, Mike and I decided to err on the side of caution;
we zero the "undefined" bits in alloc_cpumask_var_node() until all the
old cpumask functions are removed.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>