ubifs: Convert ubifs to read_folio

This is a "weak" conversion which converts straight back to using pages.
A full conversion should be performed at some point, hopefully by
someone familiar with the filesystem.

Signed-off-by: Matthew Wilcox (Oracle) <[e-mail]>

fs/ubifs/file.c

···
  * in the "sys_write -> alloc_pages -> direct reclaim path". So, in
  * 'ubifs_writepage()' we are only guaranteed that the page is locked.
  *
- * Similarly, @i_mutex is not always locked in 'ubifs_readpage()', e.g., the
+ * Similarly, @i_mutex is not always locked in 'ubifs_read_folio()', e.g., the
  * read-ahead path does not lock it ("sys_read -> generic_file_aio_read ->
- * ondemand_readahead -> readpage"). In case of readahead, @I_SYNC flag is not
+ * ondemand_readahead -> read_folio"). In case of readahead, @I_SYNC flag is not
  * set as well. However, UBIFS disables readahead.
  */
 
···
 	return err;
 }
 
-static int ubifs_readpage(struct file *file, struct page *page)
+static int ubifs_read_folio(struct file *file, struct folio *folio)
 {
+	struct page *page = &folio->page;
+
 	if (ubifs_bulk_read(page))
 		return 0;
 	do_readpage(page);
-	unlock_page(page);
+	folio_unlock(folio);
 	return 0;
 }
 
···
 }
 
 const struct address_space_operations ubifs_file_address_operations = {
-	.readpage       = ubifs_readpage,
+	.read_folio     = ubifs_read_folio,
 	.writepage      = ubifs_writepage,
 	.write_begin    = ubifs_write_begin,
 	.write_end      = ubifs_write_end,

fs/ubifs/super.c

···
 
 	/*
 	 * UBIFS provides 'backing_dev_info' in order to disable read-ahead. For
-	 * UBIFS, I/O is not deferred, it is done immediately in readpage,
+	 * UBIFS, I/O is not deferred, it is done immediately in read_folio,
 	 * which means the user would have to wait not just for their own I/O
 	 * but the read-ahead I/O as well i.e. completely pointless.
 	 *