cls_cgroup: get sk_classid only from full sockets

skb->sk could point to timewait or request socket which has no sk_classid.
Detected as "BUG: KASAN: slab-out-of-bounds in cls_cgroup_classify".

Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>