mm: remove is_longterm_pinnable_page() and reimplement folio_is_longterm_pinnable()

mm: remove is_longterm_pinnable_page() and reimplement folio_is_longterm_pinnable()

folio_is_longterm_pinnable() already exists as a wrapper function.  Now
that the whole implementation of is_longterm_pinnable_page() can be
implemented using folios, folio_is_longterm_pinnable() can be made its own
standalone function - and we can remove is_longterm_pinnable_page().

Link: https://lkml.kernel.org/r/20230614021312.34085-6-vishal.moola@gmail.com
Signed-off-by: Vishal Moola (Oracle) <vishal.moola@gmail.com>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Lorenzo Stoakes <lstoakes@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>