act_csum: fix possible use after free

tcf_csum_skb_nextlayer() / pskb_may_pull() can change skb->head, so we
must be careful not keeping pointers to previous headers.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Jamal Hadi Salim <jhs@mojatatu.com>
Cc: Grégoire Baron <baronchon@n7mm.org>
Signed-off-by: David S. Miller <davem@davemloft.net>