netfilter: xt_cgroup: Make it independent from net_cls

netfilter: xt_cgroup: Make it independent from net_cls

The xt_group matching supports the default hierarchy since commit
c38c4597e4bf3 ("netfilter: implement xt_cgroup cgroup2 path match").
The cgroup v1 matching (based on clsid) and cgroup v2 matching (based on
path) are rather independent. Downgrade the Kconfig dependency to
mere CONFIG_SOCK_GROUP_DATA so that xt_group can be built even without
CONFIG_NET_CLS_CGROUP for path matching.
Also add a message for users when they attempt to specify any clsid.

Link: https://lists.opensuse.org/archives/list/kernel@lists.opensuse.org/thread/S23NOILB7MUIRHSKPBOQKJHVSK26GP6X/
Cc: Jan Engelhardt <ej@inai.de>
Cc: Florian Westphal <fw@strlen.de>
Signed-off-by: Michal Koutný <mkoutny@suse.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>