UBIFS: fix debugging check

The debugging check which verifies that we never write outside of the file
length was incorrect, since it was multiplying file length by the page size,
instead of dividing. Fix this.

Spotted-by: hujianyang <hujianyang@huawei.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>