netfilter: x_tables: fix cgroup matching on non-full sks

While originally only being intended for outgoing traffic, commit
a00e7634 ("netfilter: x_tables: allow to use cgroup match for
LOCAL_IN nf hooks") enabled xt_cgroups for the NF_INET_LOCAL_IN hook
as well, in order to allow for nfacct accounting.

Besides being currently limited to early demuxes only, commit
a00e7634 forgot to add a check if we deal with full sockets,
i.e. in this case not with time wait sockets. TCP time wait sockets
do not have the same memory layout as full sockets, a lower memory
footprint and consequently also don't have a sk_classid member;
probing for sk_classid member there could potentially lead to a
crash.

Fixes: a00e7634 ("netfilter: x_tables: allow to use cgroup match for LOCAL_IN nf hooks")
Cc: Alexey Perevalov <a.perevalov@samsung.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>