mm/hugetlb: align cma on allocation order, not demotion order

Align the CMA area for hugetlb gigantic pages to their size, not the size
that they can be demoted to.  Otherwise there might be misaligned sections
at the start and end of the CMA area that will never be used for hugetlb
page allocations.

Link: https://lkml.kernel.org/r/20240430161437.2100295-1-fvdl@google.com


Fixes: a01f4390 ("hugetlb: be sure to free demoted CMA pages to CMA")
Signed-off-by: Frank van der Linden <fvdl@google.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Muchun Song <muchun.song@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>