Description of problem:
rmdir of a non-empty directory, that has a whiteout, will remove successfully.

Version-Release number of selected component (if applicable):
4.0.2-300.fc22.x86_64

How reproducible:
consistent

Steps to Reproduce:
```
base=$(mktemp -d)
echo ${base}

## make an overlay fs setup
mkdir -p ${base}/{upper,work,merged,mnt,lower}

## make an empty directory in the base layer
mkdir ${base}/lower/empty

## mount the base layer
sudo mount \
        -t overlay \
        -o rw,lowerdir=${base}/lower/,workdir=${base}/work/,upperdir=${base}/upper/ \
        ${base}/merged/ ${base}/mnt/

## create a whiteout
rmdir ${base}/mnt/empty

## add the dir back (on top of the whiteout)
mkdir -p ${base}/mnt/empty
touch ${base}/mnt/empty/file.txt

## now rm the non-empty directory
rmdir ${base}/mnt/empty && echo "bug here!"

## cleanup and info
sudo umount ${base}/mnt
sudo find ${base}

```

Actual results:
rmdir removes the non-empty directory and returns success

Expected results:
failure to rmdir the non-empty directory


Additional info:
Found in a docker issue report - https://github.com/docker/docker/issues/13108