f2fs: the name of a struct is wrong in a comment.

The macro SUMMARY_SIZE represents the size of the struct f2fs_summary,

instead of the size of the struct summary.

Signed-off-by: Yang Hubin <yanghb2019@lzu.edu.cn>
Signed-off-by: Qian Haolai <qianhl2023@lzu.edu.cn>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>