core: fix dm-verity auto-discovery in MountImageUnit()

core: fix dm-verity auto-discovery in MountImageUnit()

The implementation of MountImageUnit()/systemctl mount-image was
changed to use a /proc/self/fd path as the source, but that causes
the dm-verity files autodiscovery to fail, as it looks for files
in the same directory as the image.

Use the original file path when setting up dm-verity.