f2fs: fix a missing discard prefree segments

If userspace issue a fstrim with a range not involve prefree segments,
it will reuse these segments without discard. This patch fix it.

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