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 void new_curseg(struct f2fs_sb_info *sbi, int type, bool new_sec)
{
	struct curseg_info *curseg = CURSEG_I(sbi, type);
	unsigned int segno = curseg->segno;
	int dir = ALLOC_LEFT;

	write_sum_page(sbi, curseg->sum_blk,
				GET_SUM_BLOCK(sbi, segno));
	if (type == CURSEG_WARM_DATA || type == CURSEG_COLD_DATA)
		dir = ALLOC_RIGHT;

	if (test_opt(sbi, NOHEAP))
		dir = ALLOC_RIGHT;

	segno = __get_next_segno(sbi, type);
	get_new_segment(sbi, &segno, new_sec, dir);
	curseg->next_segno = segno;
	reset_curseg(sbi, type, 1);
	curseg->alloc_type = LFS;
}

static void new_curseg(struct f2fs_sb_info *sbi, int type, bool new_sec)
{
	struct curseg_info *curseg = CURSEG_I(sbi, type);
	unsigned int segno = curseg->segno;
	int dir = ALLOC_LEFT;

	write_sum_page(sbi, curseg->sum_blk,
				GET_SUM_BLOCK(sbi, segno));
	if (type == CURSEG_WARM_DATA || type == CURSEG_COLD_DATA)
		dir = ALLOC_RIGHT;

	if (test_opt(sbi, NOHEAP))
		dir = ALLOC_RIGHT;

	segno = __get_next_segno(sbi, type);
	get_new_segment(sbi, &segno, new_sec, dir);
	curseg->next_segno = segno;
	reset_curseg(sbi, type, 1);
	curseg->alloc_type = LFS;
}
