UBIFS: Change ubifsload to not read beyond the requested size

Until now ubifsload pads the destination with 0 up to a multiple of
UBIFS_BLOCK_SIZE (4KiB) while reading a file to memory. This patch
changes this behaviour to only read to the requested length. This
is either the file length or the length/size provided as parameter
to the ubifsload command.

Signed-off-by: Stefan Roese <sr@denx.de>