arm64: hugetlb: Cleanup huge_pte size discovery mechanisms

arm64: hugetlb: Cleanup huge_pte size discovery mechanisms

Not all huge_pte helper APIs explicitly provide the size of the
huge_pte. So the helpers have to depend on various methods to determine
the size of the huge_pte. Some of these methods are dubious.

Let's clean up the code to use preferred methods and retire the dubious
ones. The options in order of preference:

 - If size is provided as parameter, use it together with
   num_contig_ptes(). This is explicit and works for both present and
   non-present ptes.

 - If vma is provided as a parameter, retrieve size via
   huge_page_size(hstate_vma(vma)) and use it together with
   num_contig_ptes(). This is explicit and works for both present and
   non-present ptes.

 - If the pte is present and contiguous, use find_num_contig() to walk
   the pgtable to find the level and infer the number of ptes from
   level. Only works for *present* ptes.

 - If the pte is present and not contiguous and you can infer from this
   that only 1 pte needs to be operated on. This is ok if you don't care
   about the absolute size, and just want to know the number of ptes.

 - NEVER rely on resolving the PFN of a present pte to a folio and
   getting the folio's size. This is fragile at best, because there is
   nothing to stop the core-mm from allocating a folio twice as big as
   the huge_pte then mapping it across 2 consecutive huge_ptes. Or just
   partially mapping it.

Where we require that the pte is present, add warnings if not-present.

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>
Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
Tested-by: Luiz Capitulino <luizcap@redhat.com>
Link: https://lore.kernel.org/r/20250422081822.1836315-2-ryan.roberts@arm.com
Signed-off-by: Will Deacon <will@kernel.org>