f2fs: avoid unneeded loop in build_sit_entries

When building each sit entry in cache, firstly, we will load it from
sit page, and then check all entries in sit journal, if there is one
updated entry in journal, cover cached entry with the journaled one.

Actually, most of check operation is unneeded since we only need
to update cached entries with journaled entries in batch, so
changing the flow as below for more efficient:
1. load all sit entries into cache from sit pages;
2. update sit entries with journal.

Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>