sctp: remove the typedef sctp_errhdr_t

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

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

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