f2fs: fix count of seg_freed to make sec_freed correct

When sbi->segs_per_sec > 1, and if some segno has 0 valid blocks before
gc starts, do_garbage_collect will skip counting seg_freed++, and this
will cause seg_freed < sbi->segs_per_sec and finally skip sec_freed++.

Signed-off-by: Yunlong Song <yunlong.song@huawei.com>
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>