nilfs2: convert nilfs_mdt_get_frozen_buffer to use a folio

Remove a number of folio->page->folio conversions.

Link: https://lkml.kernel.org/r/20231016201114.1928083-15-willy@infradead.orgSigned-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Acked-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Cc: Andreas Gruenbacher <agruenba@redhat.com>
Cc: Pankaj Raghav <p.raghav@samsung.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

fs/nilfs2/mdt.c

@@ -592,17 +592,19 @@ nilfs_mdt_get_frozen_buffer(struct inode *inode, struct buffer_head *bh)
 {
 	struct nilfs_shadow_map *shadow = NILFS_MDT(inode)->mi_shadow;
 	struct buffer_head *bh_frozen = NULL;
-	struct page *page;
+	struct folio *folio;
 	int n;
 
-	page = find_lock_page(shadow->inode->i_mapping, bh->b_folio->index);
-	if (page) {
-		if (page_has_buffers(page)) {
+	folio = filemap_lock_folio(shadow->inode->i_mapping,
+			bh->b_folio->index);
+	if (!IS_ERR(folio)) {
+		bh_frozen = folio_buffers(folio);
+		if (bh_frozen) {
 			n = bh_offset(bh) >> inode->i_blkbits;
-			bh_frozen = nilfs_page_get_nth_block(page, n);
+			bh_frozen = get_nth_bh(bh_frozen, n);
 		}
-		unlock_page(page);
-		put_page(page);
+		folio_unlock(folio);
+		folio_put(folio);
 	}
 	return bh_frozen;
 }