ubifs: Convert ubifs_writepage to use a folio

We still pass the page down to do_writepage(), but ubifs_writepage()
itself is now large folio safe.  It also contains far fewer hidden calls
to compound_head().

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