fix random hangs on reboot with active CephFS mount ordering cycle

fix random hangs on reboot with active CephFS mount ordering cycle

commit 54e0b0034bd6654c566cb4ae7d4a5953c48cd1ca introduced the
"_netdev" option, for PVE 5.3. The systemd generator then correctly
resolved that in the following resulting order-dependencies:
> Wants=network-online.target
> Before=umount.target remote-fs.target
> After=remote-fs-pre.target system.slice network.target network-online.target -.mount

This worked well and all were happy. With the current systemd in 6.0
we sometimes get the local-fs ones there generated too. This is a
fallout from a try to better handling nested mount hierachies, where
a .mount unit needs to be mounter or unmounted, before or after,
respectively, the parent mount was processed. It seems that sometime
that glitches and thus a "RequireMountFor=/mnt/pve" gets thrown in
and result sometimes in the local-fs order constraints being added.

The issue now is, that one must not have ordering depends to all,
local-fs, local-fs-pre, remote-fs, remote-fs-pre, as that gets you a
ordering cycle. Systemd tries to solve that cycle by randomly
dropping one constraint and retrying. By luck this is a not so
important unit, and all goes on well. Most of the time one isn't that
lucky and something important gets dropped, for example:

> Jan 24 18:43:05 prod1 systemd[1]: sysinit.target: Found ordering cycle on systemd-timesyncd.service/stop
> Jan 24 18:43:05 prod1 systemd[1]: sysinit.target: Found dependency on systemd-tmpfiles-setup.service/stop
> Jan 24 18:43:05 prod1 systemd[1]: sysinit.target: Found dependency on local-fs.target/stop
> Jan 24 18:43:05 prod1 systemd[1]: sysinit.target: Found dependency on mnt-pve-cephfs.mount/stop
> Jan 24 18:43:05 prod1 systemd[1]: sysinit.target: Found dependency on remote-fs-pre.target/stop
> Jan 24 18:43:05 prod1 systemd[1]: sysinit.target: Found dependency on rbdmap.service/stop
> Jan 24 18:43:05 prod1 systemd[1]: sysinit.target: Found dependency on sysinit.target/stop
> Jan 24 18:43:05 prod1 systemd[1]: sysinit.target: Job remote-fs-pre.target/stop deleted to break ordering cycle starting with sysinit.target/stop

Then, most of the time the host reboot hangs for ~10 minutes, often
showing scapegoat units like the pve-ha-lrm being the cause of the
hang (even if no HA is configure >.<).

This behavior is fixed with newer systemd versions, e.g., the v244
from buster-backports, but that is not a real option for us for now.

So until 7.0 we generate the unit with the correct dependencies
directly in the ephemeral /run/ tmpfs backed systemd/system path and
start it.

While FUSE gets only the local-fs ordering constraint, it seems to cope
very well regarding such symptoms. But it _is_ racy and probably only
works due to systemd stopping it early as it has not much ordering
constraints at all.. It should be moved in the future nonetheless, as
there's a mount.fuse.ceph helper that should be not an issue.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>