cls_cgroup: factor out classid retrieval

Split out retrieving the cgroups net_cls classid retrieval into its
own function, so that it can be reused later on from other parts of
the traffic control subsystem. If there's no skb->sk, then the small
helper returns 0 as well, which in cls_cgroup terms means 'could not
classify'.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Cc: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>