ubifs: Convert ubifs_write_begin() to use a folio

Save eight calls to compound_head() by using the new folio API.
Remove a few assumptions that would break with 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>