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>

fs/squashfs/squashfs_fs_sb.h

@@ -64,7 +64,7 @@ struct squashfs_sb_info {
 	struct mutex		read_data_mutex;
 	struct mutex		meta_index_mutex;
 	struct meta_index	*meta_index;
-	z_stream		stream;
+	void			*stream;
 	__le64			*inode_lookup_table;
 	u64			inode_table;
 	u64			directory_table;