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 int __issue_discard_async(struct f2fs_sb_info *sbi,
		struct block_device *bdev, block_t blkstart, block_t blklen)
{
#ifdef CONFIG_BLK_DEV_ZONED
	if (f2fs_sb_mounted_blkzoned(sbi->sb) &&
				bdev_zoned_model(bdev) != BLK_ZONED_NONE)
		return __f2fs_issue_discard_zone(sbi, bdev, blkstart, blklen);
#endif
	return __queue_discard_cmd(sbi, bdev, blkstart, blklen);
}

static int __issue_discard_async(struct f2fs_sb_info *sbi,
		struct block_device *bdev, block_t blkstart, block_t blklen)
{
#ifdef CONFIG_BLK_DEV_ZONED
	if (f2fs_sb_mounted_blkzoned(sbi->sb) &&
				bdev_zoned_model(bdev) != BLK_ZONED_NONE)
		return __f2fs_issue_discard_zone(sbi, bdev, blkstart, blklen);
#endif
	return __queue_discard_cmd(sbi, bdev, blkstart, blklen);
}
