mm: simplify folio_page() and folio_page_idx()

Now that a single folio/compound page can no longer span memory sections
in problematic kernel configurations, we can stop using nth_page() in
folio_page() and folio_page_idx().

While at it, turn both macros into static inline functions and add kernel
doc for folio_page_idx().

Link: https://lkml.kernel.org/r/20250901150359.867252-13-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: Wei Yang <richard.weiyang@gmail.com>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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