f2fs: fix a panic caused by NULL flush_cmd_control

Mount fs with option noflush_merge, boot failed for illegal address
fcc in function f2fs_issue_flush:

        if (!test_opt(sbi, FLUSH_MERGE)) {
                ret = submit_flush_wait(sbi);
                atomic_inc(&fcc->issued_flush);   ->  Here, fcc illegal
                return ret;
        }

Signed-off-by: Yunlei He <heyunlei@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
static int __get_segment_type(struct f2fs_io_info *fio)
{
	int type = 0;

	switch (fio->sbi->active_logs) {
	case 2:
		type = __get_segment_type_2(fio);
		break;
	case 4:
		type = __get_segment_type_4(fio);
		break;
	case 6:
		type = __get_segment_type_6(fio);
		break;
	default:
		f2fs_bug_on(fio->sbi, true);
	}

	if (IS_HOT(type))
		fio->temp = HOT;
	else if (IS_WARM(type))
		fio->temp = WARM;
	else
		fio->temp = COLD;
	return type;
}

static int __get_segment_type(struct f2fs_io_info *fio)
{
	int type = 0;

	switch (fio->sbi->active_logs) {
	case 2:
		type = __get_segment_type_2(fio);
		break;
	case 4:
		type = __get_segment_type_4(fio);
		break;
	case 6:
		type = __get_segment_type_6(fio);
		break;
	default:
		f2fs_bug_on(fio->sbi, true);
	}

	if (IS_HOT(type))
		fio->temp = HOT;
	else if (IS_WARM(type))
		fio->temp = WARM;
	else
		fio->temp = COLD;
	return type;
}
