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 f2fs_wait_on_encrypted_page_writeback(struct f2fs_sb_info *sbi,
							block_t blkaddr)
{
	struct page *cpage;

	if (blkaddr == NEW_ADDR || blkaddr == NULL_ADDR)
		return;

	cpage = find_lock_page(META_MAPPING(sbi), blkaddr);
	if (cpage) {
		f2fs_wait_on_page_writeback(cpage, DATA, true);
		f2fs_put_page(cpage, 1);
	}
}

void f2fs_wait_on_encrypted_page_writeback(struct f2fs_sb_info *sbi,
							block_t blkaddr)
{
	struct page *cpage;

	if (blkaddr == NEW_ADDR || blkaddr == NULL_ADDR)
		return;

	cpage = find_lock_page(META_MAPPING(sbi), blkaddr);
	if (cpage) {
		f2fs_wait_on_page_writeback(cpage, DATA, true);
		f2fs_put_page(cpage, 1);
	}
}
