Follow-up for d120ce478d
blockdev@.target is used as a synchronization point between
the mount unit and corresponding systemd-cryptsetup@.service.
After the mentioned commit, it doesn't get a stop job enqueued
during shutdown, and thus the stop job for systemd-cryptsetup@.service
could be run before the mount unit is stopped.
Therefore, let's make blockdev@.target conflict with umount.target,
which is also what systemd-cryptsetup@.service does.
Fixes#29336
We want that cryptsetup/veritysetup devices can stick around until the
very end, as well as the users of them which might depend on
blockdev@.target for the devices. Hence leave the targets around till
the very end.
Note that their runtime is managed via StopWhenUnneeded= anyway, hence
unless their are volumes that actually survive still the very end they
target units will still be stopped.
