Fixes#43442.
A `.swap` unit can end up **active but unloadable, with no device**, and
stopping such a unit aborts PID 1.
### What it looks like
```
$ systemctl show 'dev-disk-byx2dpartlabel-primary.swap' -p LoadState,ActiveState,What
LoadState=not-found
ActiveState=active
What= # <-- no device
$ systemctl stop 'dev-disk-byx2dpartlabel-primary.swap'
systemd[1]: About to execute: /usr/sbin/swapoff # <-- no argument
systemd[1]: Caught <ABRT>, from our own process.
systemd[1]: Freezing execution.
```
PID 1 stays alive but frozen, so the machine stops answering `systemctl`
and never reboots.
### How it happens
1. systemd creates one `.swap` unit per udev device link of every device
in `/proc/swaps`.
2. A device link gets repointed at another device while the swap stays
on. `/proc/swaps` doesn't change, so nothing re-examines the unit — it
stays active.
3. The next `daemon-reload` drops all units and rebuilds them.
Enumeration from `/proc/swaps` no longer produces this name, so only the
deserialization brings the unit back: by name, with no fragment on disk.
4. `swap_load()` only calls `swap_add_extras()` for units that loaded a
fragment or are backed by `/proc/swaps`, so this one gets neither a
`What=` nor a patched `ExecContext`.
5. `swap_coldplug()` restores its active state anyway.
6. Shutdown stops it. `unit_stop()` deliberately permits stopping units
whose load failed, so it reaches `exec_spawn()` and trips the
`PrivateVarTmp=` assertion in `exec_context_serialize()`.
Yu Watanabe
c21a49cd3d