mm: convert folio_page() back to a macro

In commit 73b3294b1152 ("mm: simplify folio_page() and folio_page_idx()")
we converted folio_page() into a static inline function.  However briefly
afterwards in commit a847b17009ec ("mm: constify highmem related functions
for improved const-correctness") we had to add some nasty const-away
casting to make the compiler happy when checking const correctness.

So let's just convert it back to a simple macro so the compiler can check
const correctness properly.  There is the alternative of using a
_Generic() similar to page_folio(), but there is not a lot of benefit
compared to just using a simple macro.

Link: https://lkml.kernel.org/r/20250923140058.2020023-1-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Kiryl Shutsemau <kas@kernel.org>
Reviewed-by: SeongJae Park <sj@kernel.org>
Reviewed-by: Vishal Moola (Oracle) <vishal.moola@gmail.com>
Reviewed-by: Dev Jain <dev.jain@arm.com>
Reviewed-by: Suren Baghdasaryan <surenb@google.com>
Reviewed-by: Lance Yang <lance.yang@linux.dev>
Reviewed-by: Wei Yang <richard.weiyang@gmail.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: "Liam R. Howlett" <Liam.Howlett@oracle.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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