mpage: use b_folio in do_mpage_readpage()

Remove this conversion of a folio back to a page.

Link: https://lkml.kernel.org/r/20221215214402.3522366-13-willy@infradead.orgSigned-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

fs/mpage.c

@@ -198,7 +198,7 @@ static struct bio *do_mpage_readpage(struct mpage_readpage_args *args)
 	/*
 	 * Then do more get_blocks calls until we are done with this folio.
 	 */
-	map_bh->b_page = &folio->page;
+	map_bh->b_folio = folio;
 	while (page_block < blocks_per_page) {
 		map_bh->b_state = 0;
 		map_bh->b_size = 0;