f2fs-tools: add and export lookup_sit_in_journal

Add lookup_sit_in_journal() which is similar to lookup_nat_in_journal()
to get the raw sit entry from journal if exist.

Signed-off-by: Sheng Yong <shengyong1@xiaomi.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>