Bug 1748840

Description of problem:
If trying to loop mount a plain file, the OS will perform `losetup` automaticly in the backgroud. And that's okay.
But if that entry is added into /etc/fstab and execute `systemctl daemon-reload`, the newly created xxx.mount script generated by systemd-fstab-generator could start tmp.mount (which is disabled by default) leading the /tmp over-mounted!

Version-Release number of selected component (if applicable):
systemd-239-17.el8.x86_64

How reproducible:
always easily

Steps to Reproduce:
0. truncate --size 1G /tmp/test_img && mkfs -t xfs /tmp/test_img && mkdir -p /test
1. echo "/tmp/test_img  /test  xfs  defaults  0 0" >> /etc/fstab
2. systemctl daemon-reload
3. systemctl start /test


Actual results:
[root.0~]# tail -1 /etc/fstab 
/tmp/test_img  /test  xfs  defaults  0 0
[root.0~]# mount /test
[root.0~]# losetup 
NAME       SIZELIMIT OFFSET AUTOCLEAR RO BACK-FILE     DIO LOG-SEC
/dev/loop0         0      0         1  0 /tmp/test_img   0     512
[root.0~]# umount /test
[root.0~]# losetup 
[root.0~]# systemctl daemon-reload
[root.0~]# systemctl cat /test/
# /run/systemd/generator/test.mount
# Automatically generated by systemd-fstab-generator

[Unit]
SourcePath=/etc/fstab
Documentation=man:fstab(5) man:systemd-fstab-generator(8)
Before=local-fs.target

[Mount]
Where=/test
What=/tmp/test_img        <<<<<<<<<<
Type=xfs
[root.0~]# df -h /tmp
Filesystem                              Size  Used Avail Use% Mounted on
/dev/mapper/rhel_ibm--x3250m4--07-root   50G  3.2G   47G   7% / <<<<<<<<
[root.0~]# systemctl start /test/
Job for test.mount failed.
See "systemctl status test.mount" and "journalctl -xe" for details.
[root.0~]# df -h /tmp
Filesystem      Size  Used Avail Use% Mounted on
tmpfs           1.9G     0  1.9G   0% /tmp               <<<<<<<<<<<<<<
[root.0~]# 
[root.0~]# systemctl stop tmp.mount
[root.0~]# df -h /tmp
Filesystem                              Size  Used Avail Use% Mounted on
/dev/mapper/rhel_ibm--x3250m4--07-root   50G  3.2G   47G   7% /


Expected results:
/tmp should not be over-mounted


Additional info:
 - This problem only occurs since 8.0.0 and does't reproduce in rhel7 now
 - It works if switching plain file to loop device directly
   E.g.
   ~~~~~~~~~~~~~~
   [root.0~]# systemctl daemon-reload
   [root.0~]# systemctl cat /test/
   # /run/systemd/generator/test.mount
   # Automatically generated by systemd-fstab-generator
   
   [Unit]
   SourcePath=/etc/fstab
   Documentation=man:fstab(5) man:systemd-fstab-generator(8)
   Before=local-fs.target
   
   [Mount]
   Where=/test
   What=/dev/loop0        <<<<<<<<<<<<
   Type=xfs
   [root.0~]# df -h /tmp
   Filesystem                              Size  Used Avail Use% Mounted on
   /dev/mapper/rhel_ibm--x3250m4--07-root   50G  3.2G   47G   7% /
   [root.0~]# systemctl start /test/
   [root.0~]# df -h /tmp
   Filesystem                              Size  Used Avail Use% Mounted on
   /dev/mapper/rhel_ibm--x3250m4--07-root   50G  3.2G   47G   7% /
   [root.0~]# umount /test/
   [root.0~]# 
   ~~~~~~~~~~~~~~