nodemask: add nodes_copy()

Nodemasks API misses the plain nodes_copy() which is required in this
series.

Signed-off-by: Yury Norov [NVIDIA] <yury.norov@gmail.com>
Signed-off-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>