f2fs: add comment for conditional compilation statement

Commit af033b2a ("f2fs: guarantee journalled quota data by checkpoint")
added function is_journalled_quota() in f2fs.h, but it located outside of
_LINUX_F2FS_H macro coverage, it has been fixed with commit 0af725fc
("f2fs: fix wrong #endif").

But anyway, in order to avoid making same mistake latter, let's add single
line comment to notice which #if the last #endif is corresponding to.

Signed-off-by: Chao Yu <yuchao0@huawei.com>
[Jaegeuk Kim: Remove unnecessary empty EOL]
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>