f2fs: clean up foreground GC flow

This patch changes to check valid block number of one GCed section
directly instead of checking the number in all segments of section
one by one in order to clean up codes of foreground GC.

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