ubifs: Convert ubifs_write_end() to use a folio

Convert the incoming page pointer to a folio and use it throughout,
saving several calls to compound_head().  Also remove some PAGE_SIZE
assumptions.

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