smb: fix some warnings reported by scripts/checkpatch.pl

Fix the following warnings:

  WARNING: __always_unused or __maybe_unused is preferred over \
                                    __attribute__((__unused__))
  WARNING: Prefer __packed over __attribute__((packed))

Signed-off-by: ChenXiaoSong <chenxiaosong@kylinos.cn>
Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>

Author: ChenXiaoSong