mm/memory_hotplug: move debug_pagealloc_map_pages() into online_pages_range()

In the near future, we want to have a single way to handover PageOffline
pages to the buddy, whereby they could have:

(a) Never been exposed to the buddy before: kept PageOffline when onlining
    the memory block.
(b) Been allocated from the buddy, for example using
    alloc_contig_range() to then be set PageOffline,

Let's start by making generic_online_page()->__free_pages_core() less
special compared to ordinary page freeing (e.g., free_contig_range()),
and perform the debug_pagealloc_map_pages() call unconditionally, even
when the online callback might decide to keep the pages offline.

All pages are already initialized with PageOffline, so nobody touches them
either way.

Link: https://lkml.kernel.org/r/20241203102050.223318-1-david@redhat.com


Signed-off-by: David Hildenbrand <david@redhat.com>
Acked-by: Oscar Salvador <osalvador@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>