Squashfs: fix block size use in LZO decompressor

Sizing the buffer using block size alone is incorrect leading
to a potential buffer over-run on 4K block size file systems
(because the metadata block size is always 8K).  Srclength is
set to the maximum expected size of the decompressed block and
it is block_size or 8K depending on whether a data or metadata
block is being decompressed.

Signed-off-by: Phillip Lougher <phillip@lougher.demon.co.uk>