UBI: fix memory leak in update path

When truncating an UBI volume, UBI should allocates a PEB-sized
buffer but does not release it, which leads to memory leaks.
This patch fixes the issue.

Reported-by: Marek Skuczynski <mareksk7@gmail.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Tested-by: Marek Skuczynski <mareksk7@gmail.com>
Cc: stable@kernel.org