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 do_write_page(struct f2fs_summary *sum, struct f2fs_io_info *fio)
{
	int type = __get_segment_type(fio);
	int err;

reallocate:
	allocate_data_block(fio->sbi, fio->page, fio->old_blkaddr,
			&fio->new_blkaddr, sum, type, fio, true);

	/* writeout dirty page into bdev */
	err = f2fs_submit_page_write(fio);
	if (err == -EAGAIN) {
		fio->old_blkaddr = fio->new_blkaddr;
		goto reallocate;
	}
}

static void do_write_page(struct f2fs_summary *sum, struct f2fs_io_info *fio)
{
	int type = __get_segment_type(fio);
	int err;

reallocate:
	allocate_data_block(fio->sbi, fio->page, fio->old_blkaddr,
			&fio->new_blkaddr, sum, type, fio, true);

	/* writeout dirty page into bdev */
	err = f2fs_submit_page_write(fio);
	if (err == -EAGAIN) {
		fio->old_blkaddr = fio->new_blkaddr;
		goto reallocate;
	}
}
