sctp: Correct access to skb->{network, transport}_header

This corrects an regression introduced by "net: Use 16bits for *_headers
fields of struct skbuff" when NET_SKBUFF_DATA_USES_OFFSET is not set. In
that case sk_buff_data_t will be a pointer, however,
skb->{network,transport}_header is now __u16.

Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: David S. Miller <davem@davemloft.net>