UBI: do not allocate the memory unnecessarily

UBI reserves an LEB sized buffer for various needs. We can use this buffer
while scanning, instead of allocating another one. This patch was originally
created by Jan Luebbe <jlu@pengutronix.de>, but then he dropped it and I picked
up and tweaked a little bit.

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