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>
int lookup_journal_in_cursum(struct f2fs_journal *journal, int type,
					unsigned int val, int alloc)
{
	int i;

	if (type == NAT_JOURNAL) {
		for (i = 0; i < nats_in_cursum(journal); i++) {
			if (le32_to_cpu(nid_in_journal(journal, i)) == val)
				return i;
		}
		if (alloc && __has_cursum_space(journal, 1, NAT_JOURNAL))
			return update_nats_in_cursum(journal, 1);
	} else if (type == SIT_JOURNAL) {
		for (i = 0; i < sits_in_cursum(journal); i++)
			if (le32_to_cpu(segno_in_journal(journal, i)) == val)
				return i;
		if (alloc && __has_cursum_space(journal, 1, SIT_JOURNAL))
			return update_sits_in_cursum(journal, 1);
	}
	return -1;
}

int lookup_journal_in_cursum(struct f2fs_journal *journal, int type,
					unsigned int val, int alloc)
{
	int i;

	if (type == NAT_JOURNAL) {
		for (i = 0; i < nats_in_cursum(journal); i++) {
			if (le32_to_cpu(nid_in_journal(journal, i)) == val)
				return i;
		}
		if (alloc && __has_cursum_space(journal, 1, NAT_JOURNAL))
			return update_nats_in_cursum(journal, 1);
	} else if (type == SIT_JOURNAL) {
		for (i = 0; i < sits_in_cursum(journal); i++)
			if (le32_to_cpu(segno_in_journal(journal, i)) == val)
				return i;
		if (alloc && __has_cursum_space(journal, 1, SIT_JOURNAL))
			return update_sits_in_cursum(journal, 1);
	}
	return -1;
}
