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>
void update_meta_page(struct f2fs_sb_info *sbi, void *src, block_t blk_addr)
{
	struct page *page = grab_meta_page(sbi, blk_addr);
	void *dst = page_address(page);

	if (src)
		memcpy(dst, src, PAGE_SIZE);
	else
		memset(dst, 0, PAGE_SIZE);
	set_page_dirty(page);
	f2fs_put_page(page, 1);
}

void update_meta_page(struct f2fs_sb_info *sbi, void *src, block_t blk_addr)
{
	struct page *page = grab_meta_page(sbi, blk_addr);
	void *dst = page_address(page);

	if (src)
		memcpy(dst, src, PAGE_SIZE);
	else
		memset(dst, 0, PAGE_SIZE);
	set_page_dirty(page);
	f2fs_put_page(page, 1);
}
