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 allocate_segment_by_default(struct f2fs_sb_info *sbi,
						int type, bool force)
{
	struct curseg_info *curseg = CURSEG_I(sbi, type);

	if (force)
		new_curseg(sbi, type, true);
	else if (!is_set_ckpt_flags(sbi, CP_CRC_RECOVERY_FLAG) &&
					type == CURSEG_WARM_NODE)
		new_curseg(sbi, type, false);
	else if (curseg->alloc_type == LFS && is_next_segment_free(sbi, type))
		new_curseg(sbi, type, false);
	else if (need_SSR(sbi) && get_ssr_segment(sbi, type))
		change_curseg(sbi, type, true);
	else
		new_curseg(sbi, type, false);

	stat_inc_seg_type(sbi, curseg);
}

static void allocate_segment_by_default(struct f2fs_sb_info *sbi,
						int type, bool force)
{
	struct curseg_info *curseg = CURSEG_I(sbi, type);

	if (force)
		new_curseg(sbi, type, true);
	else if (!is_set_ckpt_flags(sbi, CP_CRC_RECOVERY_FLAG) &&
					type == CURSEG_WARM_NODE)
		new_curseg(sbi, type, false);
	else if (curseg->alloc_type == LFS && is_next_segment_free(sbi, type))
		new_curseg(sbi, type, false);
	else if (need_SSR(sbi) && get_ssr_segment(sbi, type))
		change_curseg(sbi, type, true);
	else
		new_curseg(sbi, type, false);

	stat_inc_seg_type(sbi, curseg);
}
