ubifs: Convert write_begin_slow() to use a folio

Update to new APIs, removing several calls to compound_head() and
including support for large folios.

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