buffer: use a folio in __find_get_block_slow()

Saves a call to compound_head() and may be needed to support block size >
PAGE_SIZE.

Link: https://lkml.kernel.org/r/20230612210141.730128-14-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

@@ -195,19 +195,19 @@ __find_get_block_slow(struct block_device *bdev, sector_t block)
 	pgoff_t index;
 	struct buffer_head *bh;
 	struct buffer_head *head;
-	struct page *page;
+	struct folio *folio;
 	int all_mapped = 1;
 	static DEFINE_RATELIMIT_STATE(last_warned, HZ, 1);
 
 	index = block >> (PAGE_SHIFT - bd_inode->i_blkbits);
-	page = find_get_page_flags(bd_mapping, index, FGP_ACCESSED);
-	if (!page)
+	folio = __filemap_get_folio(bd_mapping, index, FGP_ACCESSED, 0);
+	if (IS_ERR(folio))
 		goto out;
 
 	spin_lock(&bd_mapping->private_lock);
-	if (!page_has_buffers(page))
+	head = folio_buffers(folio);
+	if (!head)
 		goto out_unlock;
-	head = page_buffers(page);
 	bh = head;
 	do {
 		if (!buffer_mapped(bh))
@@ -237,7 +237,7 @@ __find_get_block_slow(struct block_device *bdev, sector_t block)
 	}
 out_unlock:
 	spin_unlock(&bd_mapping->private_lock);
-	put_page(page);
+	folio_put(folio);
 out:
 	return ret;
 }