Otherwise, the scope that the nspawn container belonging to may be
removed before the grandchild process of the machined exits and it may
be SIGKILLed.
```
[ 100.829613] systemd-machined[678]: Successfully forked off '(sd-bindmnt)' as PID 2962.
[ 100.833366] systemd-nspawn[2953]: Inner child finished, invoking payload.
[ 100.836111] (sd-bindmnt)[2962]: Skipping PR_SET_MM, as we don't have privileges.
[ 100.836401] (sd-bindmnt)[2962]: Successfully forked off '(sd-bindmnt-inner)' as PID 2964.
[ 100.846498] (sd-bindmnt)[2962]: (sd-bindmnt-inner) terminated by signal KILL.
[ 100.848846] systemd[1]: machine-TEST\x2d13\x2dNSPAWN.machinectl\x2dbind.7ye.scope: cgroup is empty
[ 100.849303] systemd[1]: machine-TEST\x2d13\x2dNSPAWN.machinectl\x2dbind.7ye.scope: Deactivated successfully.
[ 100.849317] systemd[1]: machine-TEST\x2d13\x2dNSPAWN.machinectl\x2dbind.7ye.scope: Changed running -> dead
[ 100.849752] systemd[1]: machine-TEST\x2d13\x2dNSPAWN.machinectl\x2dbind.7ye.scope: Consumed 91ms CPU time, 1.3M memory peak.
[ 100.850399] systemd-machined[678]: (sd-bindmnt) failed with exit status 1.
[ 100.850414] systemd-machined[678]: Child failed.
[ 100.854574] systemd-machined[678]: Failed to mount /tmp/marker-varlink on /tmp/marker-varlink in the namespace of machine 'TEST-13-NSPAWN.machinectl-bind.7ye': Protocol error
```
Hopefully fixes#38434.
Yu Watanabe
7b20a5cbf3