f2fs: init dirty_secmap incorrectly

section is dirty, but dirty_secmap may not set

Reported-by: Jia Yang <jiayang5@huawei.com>
Fixes: da52f8ad ("f2fs: get the right gc victim section when section has several segments")
Cc: <stable@vger.kernel.org>
Signed-off-by: Jack Qiu <jack.qiu@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>