debian/cephadm.postinst: specify --home when adduser

debian/cephadm.postinst: specify --home when adduser

quote from adduser/NEWS.Debian.gz:

>  System user home defaults to /nonexistent if --home is not specified.
>  Packages that call adduser to create system accounts should explicitly
>  specify a location for /home (see Lintian check
>  maintainer-script-lacks-home-in-adduser).

so let's follow this change in adduser. otherwise "cephadm"
would have a $HOME at `/nonexistent`.

Fixes: https://tracker.ceph.com/issues/64069
Signed-off-by: Kefu Chai <tchaikov@gmail.com>
(cherry picked from commit 90bc0369243077c2aaf67f0de2bab5810b217f4e)