alpha: define granularity hint PTE bits

Bits <6:5> of the Alpha PTE are the granularity hint, described in Table
22-3 of the Alpha Architecture Reference Manual. A hint of N marks the
PTE as one of a block of 8^N physically contiguous, naturally aligned
pages that the translation buffer may map with a single entry. With 8KB
pages that gives 64KB, 512KB and 4MB blocks.

Define the field and the helpers to encode and decode it. Nothing sets a
non-zero hint yet.

Add the hint to _PAGE_CHG_MASK so that pte_modify() preserves it. Swap
PTEs leave bits <31:0> clear, so pte_huge() is false on swap, migration
and marker entries, and page table entries above the last level keep a
zero hint where pmd_bad() would reject anything else.

Reviewed-by: Magnus Lindholm <linmag7@gmail.com>
Tested-by: Magnus Lindholm <linmag7@gmail.com>
Signed-off-by: Matt Turner <mattst88@gmail.com>
Link: https://lore.kernel.org/r/20261007-alpha-hugepages-v2-4-8e108af2d961@gmail.com
Signed-off-by: Magnus Lindholm <linmag7@gmail.com>

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