net/sched: act_ct: Use pskb_network_may_pull()

To make the filler functions more generic, use network
relative skb pulling.

Signed-off-by: Paul Blakey <paulb@mellanox.com>
Acked-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>