mm/page_alloc: fix obsolete comment in deferred_pfn_valid()

There are no architectures that can have holes in the memory map within a
pageblock since commit 859a85dd

 ("mm: remove pfn_valid_within() and
CONFIG_HOLES_IN_ZONE").  Update the corresponding comment.

Link: https://lkml.kernel.org/r/20220916072257.9639-17-linmiaohe@huawei.com
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>
Reviewed-by: Oscar Salvador <osalvador@suse.de>
Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>