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: Wanpeng Li <wanpeng.li@linux.intel.com>
Reviewed-by: Chao Yu <chao2.yu@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>