[RFC] hugetlb: add memory-hotplug notifier to only allocate for online nodes

[Swaraj Gaikwad <swarajgaikwad1925@gmail.com>]

This patch is a RFC on a proposed change to the hugetlb cgroup subsystem’s
css allocation function.

The existing hugetlb_cgroup_css_alloc() uses for_each_node() to allocate
nodeinfo for all nodes, including those which are not online yet
(or never will be). This can waste considerable memory on large-node systems.
The documentation already lists this as a TODO.

Proposed Change:
    Introduce a memory hotplug notifier that listens for MEM_ONLINE
    events. When a node becomes online, we call the same allocation function
    but insted of for_each_node(),using for_each_online_node(). This means
    memory is only allocated for nodes which are online, thus reducing waste.

Feedback Requested:
    - Where in the codebase (which file or section) is it most appropriate to
      implement and register the memory hotplug notifier for this subsystem?
    - Are there best practices or patterns for handling the notifier lifecycle,
      especially for unregistering during cgroup or subsystem teardown?
    - What are the standard methods or tools to test memory hotplug scenarios
      for cgroups? Are there ways to reliably trigger node online/offline events
      in a development environment?
    - Are there existing test cases or utilities in the kernel tree that would help
      to verify correct behavior of this change?
    - Any suggestions for implementation improvements or cleaner API usage?

Thanks,
Swaraj Gaikwad <swarajgaikwad1925@gmail.com>

Signed-off-by: Swaraj Gaikwad <swarajgaikwad1925@gmail.com>