9p: set page uptodate when required in write_end()

Commit 77469c3f570 prevented setting the page as uptodate when we wrote
the right amount of data, fix that.

Fixes: 77469c3f570 ("9p: saner ->write_end() on failing copy into non-uptodate page")
Reviewed-by: Jan Kara <jack@suse.com>
Signed-off-by: Alexander Levin <alexander.levin@verizon.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>