f2fs: do SSR for data when there is enough free space

In allocate_segment_by_default(), need_SSR() already detected it's time to do
SSR. So, let's try to find victims for data segments more aggressively in time.

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