f2fs: get the right gc victim section when section has several segments

Assume each section has 4 segment:
     .___________________________.
     |_Segment0_|_..._|_Segment3_|
     .                          .
     .                  .
     .__________.
     |_section0_|

Segment 0~2 has 0 valid block, segment 3 has 512 valid blocks.
It will fail if we want to gc section0 in this scenes,
because all 4 segments in section0 is not dirty.
So we should use dirty section bitmap instead of dirty segment bitmap
to get right victim section.

Signed-off-by: Jack Qiu <jack.qiu@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>