mm: factor out large folio handling from folio_nr_pages() into folio_large_nr_pages()

Let's factor it out into a simple helper function.  This helper will also
come in handy when working with code where we know that our folio is
large.

While at it, let's consistently return a "long" value from all these
similar functions.  Note that we cannot use "unsigned int" (even though
_folio_nr_pages is of that type), because it would break some callers that
do stuff like "-folio_nr_pages()".  Both "int" or "unsigned long" would
work as well.

Maybe in the future we'll have the nr_pages readily available for all
large folios, maybe even for small folios, or maybe for none.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: David Hildenbrand <[email protected]>
Reviewed-by: Kirill A. Shutemov <[email protected]>
Cc: Andy Lutomirks^H^Hski <[email protected]>
Cc: Borislav Betkov <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Jann Horn <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Jonathan Corbet <[email protected]>
Cc: Lance Yang <[email protected]>
Cc: Liam Howlett <[email protected]>
Cc: Lorenzo Stoakes <[email protected]>
Cc: Matthew Wilcow (Oracle) <[email protected]>
Cc: Michal Koutn <[email protected]>
Cc: Muchun Song <[email protected]>
Cc: tejun heo <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: Zefan Li <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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