mm/vmemmap: allow architectures to override how vmemmap optimization works

Architectures like powerpc will like to use different page table
allocators and mapping mechanisms to implement vmemmap optimization. 
Similar to vmemmap_populate allow architectures to implement
vmemap_populate_compound_pages

Link: https://lkml.kernel.org/r/20230724190759.483013-5-aneesh.kumar@linux.ibm.comSigned-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Christophe Leroy <christophe.leroy@csgroup.eu>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Joao Martins <joao.m.martins@oracle.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Will Deacon <will@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/sparse-vmemmap.c

@@ -358,6 +358,7 @@ int __meminit vmemmap_populate_hugepages(unsigned long start, unsigned long end,
 	return 0;
 }
 
+#ifndef vmemmap_populate_compound_pages
 /*
  * For compound pages bigger than section size (e.g. x86 1G compound
  * pages with 2M subsection size) fill the rest of sections as tail
@@ -446,6 +447,8 @@ static int __meminit vmemmap_populate_compound_pages(unsigned long start_pfn,
 	return 0;
 }
 
+#endif
+
 struct page * __meminit __populate_section_memmap(unsigned long pfn,
 		unsigned long nr_pages, int nid, struct vmem_altmap *altmap,
 		struct dev_pagemap *pgmap)