f2fs: select target segment with closer temperature in SSR mode

In SSR mode, we can allocate target segment which has different
temperature type from the type of current block, in order to avoid
mixing coldest and hottest data/node as much as possible, change
SSR allocation policy to select closer temperature for current
block prior.

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