When an automount unit ceases to exist across daemon-reload (observed:
because a generator no longer emits it), its deserialized state still
makes `automount_coldplug()` pick up the autofs pipe and resume the unit
as waiting/running. But nothing can serve that mount point anymore. The
kernel keeps blocking every process that touches the mount point until
PID 1 answers (which it won't), forever.
To fix this, fail pending requests and unmount the autofs when
coldplugging an automount unit that is not loaded anymore.
Fixes#43700
Signed-off-by: Martin Pitt <martin@amutable.com>