sctp: remove the typedef sctp_sack_variable_t

This patch is to remove the typedef sctp_sack_variable_t, and
replace with union sctp_sack_variable in the places where it's
using this typedef.

It is also to fix some indents in sctp_acked().

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