sctp: remove the typedef sctp_datahdr_t

This patch is to remove the typedef sctp_datahdr_t, and replace with
struct sctp_datahdr in the places where it's using this typedef.

It is also to use izeof(variable) instead of sizeof(type).

Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>