mm/huge_memory: add and use normal_or_softleaf_folio_pmd()

Now we have pmd_to_softleaf_folio() available to us which also raises a
CONFIG_DEBUG_VM warning if unexpectedly an invalid softleaf entry, we can
now abstract folio handling altogether.

vm_normal_folio() deals with the huge zero page (which is present), as well
as PFN map/mixed map mappings in both cases returning NULL.

Otherwise, we try to obtain the softleaf folio.

This makes the logic far easier to comprehend and has it use the standard
vm_normal_folio_pmd() path for decoding of present entries.

Finally, we have to update the flushing logic to only do so if a folio is
established.

This patch also makes the 'is_present' value more accurate - because PFN
map, mixed map and zero huge pages are present, just not present and
'normal'.

[ljs@kernel.org: avoid bisection hazard]
  Link: https://lkml.kernel.org/r/d0cc6161-77a4-42ba-a411-96c23c78df1b@lucifer.local
Link: https://lkml.kernel.org/r/c2be872d64ef9573b80727d9ab5446cf002f17b5.1774029655.git.ljs@kernel.org
Signed-off-by: Lorenzo Stoakes (Oracle) <ljs@kernel.org>
Reviewed-by: Suren Baghdasaryan <surenb@google.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Barry Song <baohua@kernel.org>
Cc: David Hildenbrand <david@kernel.org>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Lance Yang <lance.yang@linux.dev>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Nico Pache <npache@redhat.com>
Cc: Qi Zheng <zhengqi.arch@bytedance.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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