buffer: convert link_dev_buffers to take a folio

Its one caller already has a folio, so switch it to use the folio API. 
Removes a hidden call to compound_head().

Link: https://lkml.kernel.org/r/20230612210141.730128-13-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>

fs/buffer.c

@@ -907,8 +907,8 @@ struct buffer_head *alloc_page_buffers(struct page *page, unsigned long size,
 }
 EXPORT_SYMBOL_GPL(alloc_page_buffers);
 
-static inline void
-link_dev_buffers(struct page *page, struct buffer_head *head)
+static inline void link_dev_buffers(struct folio *folio,
+		struct buffer_head *head)
 {
 	struct buffer_head *bh, *tail;
 
@@ -918,7 +918,7 @@ link_dev_buffers(struct page *page, struct buffer_head *head)
 		bh = bh->b_this_page;
 	} while (bh);
 	tail->b_this_page = head;
-	attach_page_private(page, head);
+	folio_attach_private(folio, head);
 }
 
 static sector_t blkdev_max_block(struct block_device *bdev, unsigned int size)
@@ -1013,7 +1013,7 @@ grow_dev_page(struct block_device *bdev, sector_t block,
 	 * run under the folio lock.
 	 */
 	spin_lock(&inode->i_mapping->private_lock);
-	link_dev_buffers(&folio->page, bh);
+	link_dev_buffers(folio, bh);
 	end_block = folio_init_buffers(folio, bdev,
 			(sector_t)index << sizebits, size);
 	spin_unlock(&inode->i_mapping->private_lock);