f2fs: remove unnecessary initialization in xattr.c

These variables will be explicitly assigned before use,
so there is no need to initialize.

Signed-off-by: Liu Song <liu.song11@zte.com.cn>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>