sctp: remove the typedef sctp_disposition_t

This patch is to remove the typedef sctp_disposition_t, and
replace with enum sctp_disposition in the places where it's
using this typedef.

It's also to fix the indent for many functions' defination.

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