sctp: fix the name of struct sctp_shutdown_chunk_t

This patch is to fix the name of struct sctp_shutdown_chunk_t
, replace with struct sctp_initack_chunk in the places where
it's using it.

It is also to fix some indent problem.

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