mm: Add folio reference count functions

mm: Add folio reference count functions

These functions mirror their page reference counterparts.  Also add
the kernel-doc to the mm-api and correct the return type of
page_ref_add_unless() to bool.  No change to generated code.

Signed-off-by: Matthew Wilcox (Oracle) <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>
Acked-by: Jeff Layton <[email protected]>
Acked-by: Kirill A. Shutemov <[email protected]>
Acked-by: Vlastimil Babka <[email protected]>
Reviewed-by: William Kucharski <[email protected]>
Reviewed-by: David Howells <[email protected]>
Acked-by: Mike Rapoport <[email protected]>