ubifs: Set page uptodate in the correct place

Page cache reads are lockless, so setting the freshly allocated page
uptodate before we've overwritten it with the data it's supposed to have
in it will allow a simultaneous reader to see old data.  Move the call
to SetPageUptodate into ubifs_write_end(), which is after we copied the
new data into the page.

Fixes: 1e51764a

 ("UBIFS: add new flash file system")
Cc: stable@vger.kernel.org
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>