squashfs: fix locking bug in zlib wrapper

Fix locking bug in zlib wrapper introduced by recent decompressor changes.
Signed-off-by: Phillip Lougher <phillip@lougher.demon.co.uk>

fs/squashfs/zlib_wrapper.c

@@ -128,8 +128,9 @@ static int zlib_uncompress(struct squashfs_sb_info *msblk, void **buffer,
 		goto release_mutex;
 	}
 
+	length = stream->total_out;
 	mutex_unlock(&msblk->read_data_mutex);
-	return stream->total_out;
+	return length;
 
 release_mutex:
 	mutex_unlock(&msblk->read_data_mutex);