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>

fs/f2fs/f2fs.h

@@ -3673,4 +3673,4 @@ static inline bool is_journalled_quota(struct f2fs_sb_info *sbi)
 	return false;
 }
 
-#endif
+#endif /* _LINUX_F2FS_H */