mm: add softleaf_is_valid_pmd_entry(), pmd_to_softleaf_folio()

Separate pmd_is_valid_softleaf() into separate components, then use the
pmd_is_valid_softleaf() predicate to implement pmd_to_softleaf_folio().

This returns the folio associated with a softleaf entry at PMD level. It
expects this to be valid for a PMD entry.

If CONFIG_DEBUG_VM is set, then assert on this being an invalid entry, and
either way return NULL in this case.

This lays the ground for further refactorings.

Link: https://lkml.kernel.org/r/b677592596274fa3fd701890497948e4b0e07cec.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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