f2fs: fix to correct no_fggc_candidate

There may be extreme case as below:

For one section contains one segment, and there are total 100 segments
with 10% over-privision ratio in f2fs partition, fggc_threshold will
be rounded down to 460 instead of 460.8 as below caclulation:

sbi->fggc_threshold = div_u64((u64)(main_count - ovp_count) *
			BLKS_PER_SEC(sbi), (main_count - resv_count));

If section usage is as:
60 segments which contain 460 valid blocks
40 segments which contain 462 valid blocks

As valid block number in all sections is large than fggc_threshold, so
none of them will be chosen as candidate due to incorrect fggc_threshold.

Let's just soften the term of choosing foreground GC candidates.

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