Description of problem:
When PrivateDevices=true is used in a service like postfix, then it makes the service to fail on starting. It happens even when SELinux is in permissive mode. 


Version-Release number of selected component (if applicable):
systemd-243-4.gitef67743.fc31.s390x
postfix-3.4.7-1.fc31.s390x


How reproducible:
100%

Steps to Reproduce:
1. install F-31
2. install postfix
3. systemctl start postfix

Actual results:
Nov 06 03:54:53 devel7.s390.bos.redhat.com systemd[1]: Starting Postfix Mail Transport Agent...
Nov 06 03:54:53 devel7.s390.bos.redhat.com postfix[12641]: /usr/libexec/postfix/postfix-script: line 127: /dev/null: Operation not permitted
Nov 06 03:54:53 devel7.s390.bos.redhat.com postfix/postfix-script[12648]: fatal: the Postfix mail system is already running
Nov 06 03:54:54 devel7.s390.bos.redhat.com systemd[1]: postfix.service: Control process exited, code=exited, status=1/FAILURE
Nov 06 03:54:54 devel7.s390.bos.redhat.com systemd[1]: postfix.service: Failed with result 'exit-code'.
Nov 06 03:54:54 devel7.s390.bos.redhat.com systemd[1]: Failed to start Postfix Mail Transport Agent.


Expected results:
postfix started


Additional info:
This is very similar to bug 1746413 when dbus-broker couldn't be started with SELinux in enforcing mode.