sctp: remove the typedef sctp_sackhdr_t

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

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