Squashfs: move zlib decompression wrapper code into a separate file

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

diff --cc fs/squashfs/Makefile
Simple merge

diff --cc fs/squashfs/block.c
Simple merge

diff --cc fs/squashfs/squashfs.h
Simple merge

diff --cc fs/squashfs/zlib_wrapper.c
Simple merge