sctp: load transport header after sk_filter

Do not cache pointers into the skb linear segment across sk_filter.
The function call can trigger pskb_expand_head.

Signed-off-by: Willem de Bruijn <willemb@google.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>