squashfs: fix potential buffer over-run on 4K block file systems

Sizing the buffer based on block size is incorrect, leading
to a potential buffer over-run on 4K block size file systems
(because the metadata block size is always 8K).  This bug
doesn't seem have triggered because 4K block size file systems
are not default, and also because metadata blocks after
compression tend to be less than 4K.

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