sctp: remove the typedef sctp_sctphdr_t

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

It is also to fix some indents and 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>