buffer: convert init_page_buffers() to folio_init_buffers()

Use the folio API and pass the folio from both callers.  Saves a hidden
call to compound_head().

Link: https://lkml.kernel.org/r/20230612210141.730128-12-willy@infradead.org


Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Andreas Gruenbacher <agruenba@redhat.com>
Cc: Bob Peterson <rpeterso@redhat.com>
Cc: Hannes Reinecke <hare@suse.com>
Cc: Luis Chamberlain <mcgrof@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>