f2fs: don't need to collect dirty sit entries and flush journal when there's no dirty sit entries

 Don't need to collect dirty sit entries and flush sit journal to sit
 entries when there's no dirty sit entries. This patch check dirty_sentries
 earlier just like flush_nat_entries.
Signed-off-by: NWanpeng Li <wanpeng.li@linux.intel.com>
Reviewed-by: NChao Yu <chao2.yu@samsung.com>
Signed-off-by: NJaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/segment.c

@@ -1733,6 +1733,9 @@ void flush_sit_entries(struct f2fs_sb_info *sbi, struct cp_control *cpc)
 	mutex_lock(&curseg->curseg_mutex);
 	mutex_lock(&sit_i->sentry_lock);
 
+	if (!sit_i->dirty_sentries)
+		goto out;
+
 	/*
 	 * add and account sit entries of dirty bitmap in sit entry
 	 * set temporarily
@@ -1747,9 +1750,6 @@ void flush_sit_entries(struct f2fs_sb_info *sbi, struct cp_control *cpc)
 	if (!__has_cursum_space(sum, sit_i->dirty_sentries, SIT_JOURNAL))
 		remove_sits_in_journal(sbi);
 
-	if (!sit_i->dirty_sentries)
-		goto out;
-
 	/*
 	 * there are two steps to flush sit entries:
 	 * #1, flush sit entries to journal in current cold data summary block.