ubifs: Convert populate_page() to take a folio

Both callers now have a folio, so pass it in.  This function contains
several assumptions that folios are not large.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>