Squashfs: factor out remaining zlib dependencies into separate wrapper file

Move zlib buffer init/destroy code into separate wrapper file.  Also
make zlib z_stream field a void * removing the need to include zlib.h
for most files.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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