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>
bool exist_trim_candidates(struct f2fs_sb_info *sbi, struct cp_control *cpc)
{
	__u64 trim_start = cpc->trim_start;
	bool has_candidate = false;

	mutex_lock(&SIT_I(sbi)->sentry_lock);
	for (; cpc->trim_start <= cpc->trim_end; cpc->trim_start++) {
		if (add_discard_addrs(sbi, cpc, true)) {
			has_candidate = true;
			break;
		}
	}
	mutex_unlock(&SIT_I(sbi)->sentry_lock);

	cpc->trim_start = trim_start;
	return has_candidate;
}

bool exist_trim_candidates(struct f2fs_sb_info *sbi, struct cp_control *cpc)
{
	__u64 trim_start = cpc->trim_start;
	bool has_candidate = false;

	mutex_lock(&SIT_I(sbi)->sentry_lock);
	for (; cpc->trim_start <= cpc->trim_end; cpc->trim_start++) {
		if (add_discard_addrs(sbi, cpc, true)) {
			has_candidate = true;
			break;
		}
	}
	mutex_unlock(&SIT_I(sbi)->sentry_lock);

	cpc->trim_start = trim_start;
	return has_candidate;
}
