UBI: fix buffer padding

Instead of correctly pad the buffer wich we are writing to the
eraseblock during update, we used weird construct:

memset(buf + len, 0xFF, len - len);

Fix this.

Signed-off-by: Kyungmin Park <kmpark@infradead.org>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>