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 add_sit_entry(unsigned int segno, struct list_head *head)
{
	struct sit_entry_set *ses;
	unsigned int start_segno = START_SEGNO(segno);

	list_for_each_entry(ses, head, set_list) {
		if (ses->start_segno == start_segno) {
			ses->entry_cnt++;
			adjust_sit_entry_set(ses, head);
			return;
		}
	}

	ses = grab_sit_entry_set();

	ses->start_segno = start_segno;
	ses->entry_cnt++;
	list_add(&ses->set_list, head);
}

static void add_sit_entry(unsigned int segno, struct list_head *head)
{
	struct sit_entry_set *ses;
	unsigned int start_segno = START_SEGNO(segno);

	list_for_each_entry(ses, head, set_list) {
		if (ses->start_segno == start_segno) {
			ses->entry_cnt++;
			adjust_sit_entry_set(ses, head);
			return;
		}
	}

	ses = grab_sit_entry_set();

	ses->start_segno = start_segno;
	ses->entry_cnt++;
	list_add(&ses->set_list, head);
}
