mm/mm_init: handle alloc_percpu failure in free_area_init_core_hotplug

We miss a failed allocation check for pgdat->per_cpu_nodestats, which
results in a NULL deref when we offset into the per-cpu area.

Propagate -ENOMEM up the stack and leave per_cpu_nodestats pointing
at boot_nodestats so a later online can retry the allocation.

hotadd_init_pgdat() returns NULL on failure, which __try_online_node()
already maps to -ENOMEM.

On failure nothing needs to be unwound:
  - the node is never marked online
  - per_cpu_nodestats is left pointing at boot_nodestats
  - __add_memory_resource() cleans up pending memblock resources
  - later online attempts retry the per_cpu_nodestats allocation

Reported-by: Sashiko <sashiko-bot@kernel.org>
Link: https://sashiko.dev/#/patchset/20260627202243.758289-1-gourry%40gourry.net
Fixes: 75ef71840539 ("mm, vmstat: add infrastructure for per-node vmstats")
Signed-off-by: Gregory Price <gourry@gourry.net>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Link: https://patch.msgid.link/20260701221613.2818148-1-gourry@gourry.net
Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>