cephfs: update reminder for systemd_netmount removal

cephfs: update reminder for systemd_netmount removal

Commit d9ece228fbbbf0eb575c8a067a70ed7023078f84 introduced the workaround with
using systemd units and 25e222ca0d96a0da0d491e749b437d2822215e9e re-used the
functionality for fuse-mounts too.

The latter commit suggests to switch to using mount.fuse.ceph for the '_netdev'
option, but it doesn't seem to work:

 root@pve701 / # mount -t fuse.ceph 10.10.10.11,10.10.10.12,10.10.10.13:/ /mnttest/fuse -o 'ceph.id=admin,ceph.keyfile=/etc/pve/priv/ceph/cephfs.secret,ceph.conf=/etc/pve/ceph.conf,_netdev'
 ceph-fuse[20729]: starting ceph client
 2021-06-15T14:22:00.631+0200 7f995f878080 -1 init, newargv = 0x55e09fc11a40 newargc=11
 ceph-fuse[20729]: starting fuse
 root@pve701 / # mount -t ceph 10.10.10.11,10.10.10.12,10.10.10.13:/ /mnttest/normal -o 'name=admin,secretfile=/etc/pve/priv/ceph/cephfs.secret,conf=/etc/pve/ceph.conf,_netdev'
 root@pve701 / # mount | grep mnttest
 ceph-fuse on /mnttest/fuse type fuse.ceph-fuse (rw,nosuid,nodev,relatime,user_id=0,group_id=0,allow_other)
 10.10.10.11,10.10.10.12,10.10.10.13:/ on /mnttest/normal type ceph (rw,relatime,name=admin,secret=<hidden>,acl,_netdev)

Also, the return value is not propagated by mount.fuse.ceph, meaning the output
would need to be parsed...

 root@pve701 ~ # mount -t fuse.ceph 10.10.10.11,10.10.10.12,10.10.10.13:/ /mnttest/fuse -o 'ceph.id=admin,ceph.keyfile=/etc/pve/priv/ceph/cephfs.secret,ceph.conf=/etc/pve/ceph.conf,_netdev'
 2021-06-15T14:42:56.326+0200 7f634edae080 -1 init, newargv = 0x560cdb5e0a40 newargc=11
 ceph-fuse[34480]: starting ceph client
 fuse: mountpoint is not empty
 fuse: if you are sure this is safe, use the 'nonempty' mount option
 ceph-fuse[34480]: fuse failed to start
 2021-06-15T14:42:56.338+0200 7f634edae080 -1
 fuse_mount(mountpoint=/mnttest/fuse) failed.
 Mount failed with status code: 5
 root@pve701 ~ # echo $?
 0

Signed-off-by: Fabian Ebner <f.ebner@proxmox.com>