f2fs: avoid redundant f2fs_flush after remount

create_flush_cmd_control will create redundant issue_flush_thread after each
remount with flush_merge option.

Signed-off-by: Yunlong Song <yunlong.song@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>