mm/bootmem_info: avoid using sparse_decode_mem_map()

mm/bootmem_info: avoid using sparse_decode_mem_map()

With SPARSEMEM_VMEMMAP, we can just do a pfn_to_page().  It is not super
clear whether the start_pfn is properly aligned ...  so let's just make
sure it is properly aligned to the start of the section.

We will soon might try to remove the bootmem info completely, for now,
just keep it working as is.

Link: https://lkml.kernel.org/r/20260320-sparsemem_cleanups-v2-8-096addc8800d@kernel.org
Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
Reviewed-by: Lorenzo Stoakes (Oracle) <ljs@kernel.org>
Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Sidhartha Kumar <sidhartha.kumar@oracle.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: Wei Xu <weixugc@google.com>
Cc: Yuanchu Xie <yuanchu@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>