Description of problem:
I am using LVM root. Once the build is finished, the root is kept mounted. But surprisingly, the DNF cache is unmounted:

```
Executing(%clean): /bin/sh -e /var/tmp/rpm-tmp.yB21KM
+ umask 022
+ cd /builddir/build/BUILD
+ cd rubygem-hpricot-0.8.6
+ /usr/bin/rm -rf /builddir/build/BUILDROOT/rubygem-hpricot-0.8.6-13.fc26.x86_64
+ exit 0
DEBUG: Child return code was: 0
DEBUG: Copying packages to result dir
Finish: rpmbuild rubygem-hpricot-0.8.6-13.fc26.src.rpm
Finish: build phase for rubygem-hpricot-0.8.6-13.fc26.src.rpm
INFO: Done(rubygem-hpricot-0.8.6-13.fc26.src.rpm) Config(fedora-rawhide-x86_64) 0 minutes 33 seconds
INFO: Results and/or logs in: /var/lib/mock/fedora-rawhide-x86_64/result
Finish: run
DEBUG: kill orphans
DEBUG: child environment: None
DEBUG: Executing command: ['/bin/umount', '-n', '/var/lib/mock/fedora-rawhide-x86_64/root/var/cache/dnf/'] with env {'TERM': 'vt100', 'SHELL': '/bin/sh', 'HOME': '/builddir', 'HOSTNAME': 'mock', 'PATH': '/usr/bin:/bin:/usr/sbin:/sbin', 'LANG': 'C.UTF-8'} and shell False
DEBUG: Child return code was: 0
```

This is unexpected. All the root is keep mounted, why not the DNF cache?


Version-Release number of selected component (if applicable):
$ rpm -q mock
mock-1.3.3-1.fc26.noarch


How reproducible:


Steps to Reproduce:
1.
2.
3.

Actual results:
DNF cache is unmounted at the end of build.


Expected results:
DNF cache is kept mounted similarly to the rest of the build root.


Additional info:
This is my mock configuration:

```
$ cat ~/.config/mock.cfg 
# By default Mock use simple chroot(1). When you set this to True
# it will use systemd-nspawn(1)
config_opts['use_nspawn'] = True
# If you're using nspawn, then by default networking will be turned off
# for rpmbuild.  This helps ensure more reproducible builds.
config_opts['rpmbuild_networking'] = True


# You can configure Yum, DNF, rpm and rpmbuild executable paths if you need to
# use different versions that the system-wide ones
config_opts['yum_command'] = '/usr/bin/yum-deprecated'

# if you want to speed up the package installation and the build process, mock
# can use nosync library to skip fsync and related calls from programs called
# from within mock. It needs nosync library to be installed and for multilib
# target, it requires both architectures of nosync to be present. If these
# conditions aren't met, it won't be enabled
config_opts['nosync'] = True


#config_opts['plugin_conf']['root_cache_enable'] = False


# lvm_root plugin is not enabled by default and is distributed in separate
# subpackage mock-lvm. If you want to use it, it's recommended to disable the
# root_cache plugin, otherwise you'd be caching twice.
config_opts['plugin_conf']['lvm_root_enable'] = True
# config_opts['plugin_conf']['lvm_root_opts'] = {}
# You need to give it a volume group with sufficient space. It won't touch any
# of the existing logical volumes, so you can use the same volume group you use
# for other purposes. It requires a name of the VG (not device path).
# config_opts['plugin_conf']['lvm_root_opts']['volume_group'] = 'my_vg'
# You need to specify the size that will mock's thinpool occupy. For regular
# packages with small set of dependencies, 2G should suffice. For large packages
# such as libreoffice, you should set it to at least 5 GB otherwise you may run
# out of space and the build process will be blocked
# config_opts['plugin_conf']['lvm_root_opts']['size'] = '2G'
# You can specify alternative pool metadata size, format is the same as size.
# Default value is determined by lvcreate based on size
# config_opts['plugin_conf']['lvm_root_opts']['poolmetadatasize'] = None
# When thin pool utilization is over 90% mock will refuse to continue.
# Because once it reach 100% utilization weird things will happens.
# config_opts['plugin_conf']['lvm_root_opts']['check_size'] = True
config_opts['plugin_conf']['lvm_root_opts'] = {
    'volume_group': 'vg_fedora',
    'size': '20G',
    'pool_name': 'mock',
}
```