mm/numa: Introduce nearest_node_nodemask()

Introduce the new helper nearest_node_nodemask() to find the closest
node in a specified nodemask from a given starting node.

Returns MAX_NUMNODES if no node is found.

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